Detect musl by probing filesystem for native builds

LLVM's default target triple on Alpine reports x86_64-unknown-linux-gnu
instead of musl, causing the embedded LLD path to set the wrong dynamic
linker (/lib64/ld-linux-x86-64.so.2 instead of /lib/ld-musl-x86_64.so.1).
Binaries linked this way fail with "not found" because the interpreter
doesn't exist on Alpine.

For native compilation, probe for the musl dynamic linker on the
filesystem rather than trusting the triple. Cross-compilation still
relies on the triple since the host filesystem isn't relevant.

Author: SeanTAllen

src/libponyc/codegen/genexe.cc

@@ -302,10 +302,39 @@ static const char* elf_emulation(compile_t* c)
   }
 }
 
+static bool file_exists(const char* path)
+{
+  struct stat st;
+  return stat(path, &st) == 0;
+}
+
+// Check whether the host is actually musl. LLVM's default target triple
+// may report "gnu" on musl systems (e.g., Alpine's LLVM reports
+// x86_64-unknown-linux-gnu). For native compilation, probe the filesystem
+// rather than trusting the triple.
+static bool host_is_musl(compile_t* c, llvm::Triple::ArchType arch)
+{
+  if(is_cross_compiling(c))
+    return false;
+
+  const char* musl_linker = NULL;
+  switch(arch)
+  {
+    case llvm::Triple::x86_64:  musl_linker = "/lib/ld-musl-x86_64.so.1"; break;
+    case llvm::Triple::aarch64: musl_linker = "/lib/ld-musl-aarch64.so.1"; break;
+    case llvm::Triple::riscv64: musl_linker = "/lib/ld-musl-riscv64.so.1"; break;
+    case llvm::Triple::arm:
+    case llvm::Triple::thumb:   musl_linker = "/lib/ld-musl-arm.so.1"; break;
+    default: return false;
+  }
+
+  return file_exists(musl_linker);
+}
+
 static const char* dynamic_linker_path(compile_t* c)
 {
   llvm::Triple triple(c->opt->triple);
-  bool is_musl = triple.isMusl();
+  bool is_musl = triple.isMusl() || host_is_musl(c, triple.getArch());
 
   switch(triple.getArch())
   {
@@ -345,12 +374,6 @@ static const char* system_triple(compile_t* c)
   return stringtab(result.c_str());
 }
 
-static bool file_exists(const char* path)
-{
-  struct stat st;
-  return stat(path, &st) == 0;
-}
-
 static const char* find_libc_crt_dir(const char* sysroot,
   const char* sys_triple)
 {