gpu: aarch64-macos cross compilation support about mach

[slimsag]

After updating our SDK to macOS 12.0 Monterey, it seems we face a few version incompatibilities with Zig's libc version which is on the older macOS 11 version. One example:

slimsag@hexops-ci gpu % SDK_PATH="$(pwd)/../.." zig build run-dawn-example -Dtarget=aarch64-macos   
error(link): undefined reference to symbol '___isPlatformVersionAtLeast'
error(link):   first referenced in '/Users/slimsag/Desktop/hexops/mach/gpu/zig-cache/o/3bd047a77c4b38019657965ff1fa5151/libdawn-native.a(/Users/slimsag/Desktop/hexops/mach/gpu/zig-cache/o/23eba02466d5f0700a75334aa5662ea4/BackendMTL.o)'
thread 7268393 panic: attempt to unwrap error: UndefinedSymbolReference

Another example:

Zig's sys/cdefs.h on macOS targets does not define things like __kernel_ptr_semantics as empty, which leads to type definitions in e.g. IOKit/IOTypes.h becoming syntactically invalid (macro doesn't expand to nothing):

typedef mach_vm_address_t       IOVirtualAddress __kernel_ptr_semantics;

[slimsag]

Curiously, even after updating Zig's libc headers to reflect macOS 12 in ziglang/zig#10200 - the linking failure did not go away:

error(link): undefined reference to symbol '___isPlatformVersionAtLeast'
error(link):   first referenced in '/Users/slimsag/Desktop/hexops/mach/gpu/zig-cache/o/a4898ce0dd88be1c63f03f62251f9adb/libdawn-native.a(/Users/slimsag/Desktop/hexops/mach/gpu/zig-cache/o/22c50fc6233f5284bfe9a440d8943d61/BackendMTL.o)'
thread 7413503 panic: attempt to unwrap error: UndefinedSymbolReference

isPlatformVersionAtLeast is referenced in every Dawn/WebGPU binary compiled with Zig natively, but is not found when cross-compiling.

isPlatformVersionAtLeast is not defined in Dawn's source code, nor in any framework in the macOS 12 SDK so far as I can see. I've searched all source files and binaries, it's not there. Where does this thing come from?

[slimsag]

Looks like LLVM inserts it: https://reviews.llvm.org/D90367

Speculating, I suspect that when compiling natively clang's compiler-rt (which defines this symbol) is linked in - while when cross compiling Zig's port of compiler-rt does not have this as it was a relatively recent addition (Oct 2020) ziglang/zig#1504

Maybe we need to port this to Zig's compiler-rt library.

[slimsag]

Filed ziglang/zig#10227 and working on a PR for Zig

[slimsag]

ziglang/zig#10232

[slimsag]

Confirmed fixed.