Fix build on arm64ec.

[madewokherd]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Fixes the build on arm64ec, which reports itself as x86_64 even though it's an arm architecture, by disabling code sections with x86_64 assembly on arm64ec.

[cgutman]

What compiler are you using that sets __arm64ec__?

[madebr]

What are the errors you're seeing? I remember successfully building a arm64ec SDL3.

[madewokherd]

What compiler are you using that sets __arm64ec__?

llvm-mingw

[madewokherd]

What are the errors you're seeing? I remember successfully building a arm64ec SDL3.

[  7%] Building C object CMakeFiles/SDL3-shared.dir/src/atomic/SDL_spinlock.c.obj
/home/meh/wine-mono/SDL3/src/atomic/SDL_spinlock.c:157:13: error: unrecognized instruction mnemonic
  157 |             SDL_CPUPauseInstruction();
      |             ^
/home/meh/wine-mono/SDL3/include/SDL3/SDL_atomic.h:359:60: note: expanded from macro 'SDL_CPUPauseInstruction'
  359 |     #define SDL_CPUPauseInstruction() __asm__ __volatile__("pause\n")  /* Some assemblers can't do REP NOP, s...
      |                                                            ^
<inline asm>:1:2: note: instantiated into assembly here
    1 |         pause
      |         ^
1 error generated.

and later:

[  9%] Building C object CMakeFiles/SDL3-shared.dir/src/cpuinfo/SDL_cpuinfo.c.obj
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:174:7: error: unknown register name '%rax' in asm
  174 |     : "%rax", "%rcx"
      |       ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:294:13: error: invalid output constraint '=a' in asm
  294 |             cpuid(0, a, b, c, d);
      |             ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:297:17: error: invalid output constraint '=a' in asm
  297 |                 cpuid(1, a, b, c, d);
      |                 ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:308:31: error: invalid output constraint '=a' in asm
  308 |                             : "=a"(a)
      |                               ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:597:9: error: invalid output constraint '=a' in asm
  597 |         cpuid(7, a, b, c, d);
      |         ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:619:9: error: invalid output constraint '=a' in asm
  619 |         cpuid(7, a, b, c, d);
      |         ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:688:13: error: invalid output constraint '=a' in asm
  688 |             cpuid(0x00000000, a, b, c, d);
      |             ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:867:9: error: invalid output constraint '=a' in asm
  867 |         cpuid(0x00000001, a, b, c, d);
      |         ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:870:9: error: invalid output constraint '=a' in asm
  870 |         cpuid(0x80000005, a, b, c, d);
      |         ^
/home/meh/wine-mono/SDL3/src/cpuinfo/SDL_cpuinfo.c:247:11: note: expanded from macro 'cpuid'
  247 |         : "=a"(a), "=S"(b), "=c"(c), "=d"(d) \
      |           ^
9 errors generated.

[icculus]

The compiler is defining __x86_64__ and also __arm64ec__...? That seems unexpected.

[madewokherd]

It is an odd choice, but it's consistent with how msvc does it (although they use _M_AMD64 and _M_ARM64EC).

[slouken]

Yeah, this platform is native ARM64 and interprets x86_64 instructions.

[icculus]

Well then this seems reasonable to merge...?

[slouken]

Merged, thanks!