[cpullvm] Set CC/CXX in the environment for Windows builds

[jonathonpenix]

There's a few different aims here:

• Setting CMAKE_C_COMPILER, etc. directly can be a bit confusing when looking through our logs. That value is not propagated when we add multilib and the runtimes as external CMake projects so these projects report MSVC as CMAKE_C[XX]_COMPILER. We don't use MSVC in these subprojects (we manually pass in/use the just-built LLVM toolchain) so there shouldn't be any functional change here, but it should hopefully avoid confusion if/when people look through logs.

  So, I think it is best we avoid recommending this if we can.

• This also aligns our Windows builds closer to what we recommend for Linux.

With this change, I think we can also consolidate our build instructions a bit to make them (hopefully) more clear/concise.

[jonathonpenix]

Examples from our logs:

From one of our nightly runs:

[6053/6556] Performing configure step for 'multilib-picolibc'
-- The C compiler identification is MSVC 19.44.35225.0
-- The CXX compiler identification is MSVC 19.44.35225.0

From building with this change:

[6053/6556] Performing configure step for 'multilib-picolibc'
-- The C compiler identification is Clang 19.1.5 with MSVC-like command-line
-- The CXX compiler identification is Clang 19.1.5 with MSVC-like command-line

Again this is really a cosmetic change, but it makes us a bit more consistent/hopefully preempts some confusion around this.

[jonathonpenix]

Successful Windows builds (eld failures unrelated):

• Nightly: https://github.com/qualcomm/cpullvm-toolchain/actions/runs/24491797450?pr=317
• Native runtime nightly: https://github.com/qualcomm/cpullvm-toolchain/actions/runs/24491797442?pr=317

[apazos]

Is this an issue only on Windows? I think users might be more familiar to setting -DCMAKE_C_COMPILER, -DCMAKE_CXX_COMPILER, so should we keep them and add a note about Windows?

[jonathonpenix]

It's not specific to Windows, it's just we already use CC/CXX on Linux in our build scripts, etc.

There isn't a particularly good reason why we weren't doing this on Windows before--it's just that when I was rushing through things I wasn't sure if CMake was consistent about this on Windows

I'm also not necessarily opposed to going through CMAKE_C_COMPILER (we can always pass this through to subprojects or something), but I think it's worth being consistent between whatever we recommend for Linux/Windows