machine files: add [compilers] section for hermetic toolchain configuration

[Prince781]

Adds a new [compilers] section to native and cross machine files that allows declaring compiler configuration explicitly, rather than relying on Meson to auto-detect it from a binary. This is primarily useful for hermetic toolchains where the compiler and its subprograms require a custom execution environment not present on the build host.

When a language is declared in this section, the user can override compiler properties that Meson would infer by running <compiler> --version. <lang>.type allows controlling the compiler class. <lang>.version is an optional override.

The remaining keys -- sysroot, no-default-includes, system-include-dirs, tool-search-paths, and subprocess-interpreter -- translate to compiler flags at setup time so that the binary entries stay clean:

• sysroot -> --sysroot=<path> (GCC/Clang); ignored for MSVC-family
• no-default-includes -> -nostdinc / /X
• system-include-dirs -> -isystem <dir> / /imsvc <dir>
• tool-search-paths -> -B <dir> (GCC/Clang)
• subprocess-interpreter -> GCC only: Meson generates thin cc1/cc1plus/lto1 wrapper scripts in the build scratch directory that invoke the real subprograms via the specified ELF interpreter, making the build fully cacheable by ccache without requiring -wrapper or LD_LIBRARY_PATH.

Example for a self-contained GCC 12.2.0 bundled under sdk/:

[constants]
_sdk     = '@GLOBAL_SOURCE_ROOT@' / 'sdk'
_gcc     = _sdk / 'gcc-12.2.0'
_runtime = _sdk / 'runtime'    # ELF loader and companion shared libraries

[binaries]
c   = _gcc / 'bin' / 'x86_64-linux-gnu-gcc'
cpp = _gcc / 'bin' / 'x86_64-linux-gnu-g++'

[compilers]
c.type    = 'gcc'
c.version = '12.2.0'
c.subprocess-interpreter = [_runtime / 'lib64' / 'ld-linux-x86-64.so.2',
                              '--library-path', _runtime / 'lib64']

cpp.type    = 'gcc'
cpp.version = '12.2.0'
cpp.subprocess-interpreter = c.subprocess-interpreter

Fixes #12612

[gurchetansingh]

You might want to take a look at #15462 and #14147 for other takes on a similar problem.

From a brief skim, wouldn't it be nice to be able to download the SDK and compilers via a wrap file? That's what my proposal does:

[[wrap]]
name = "android_ndk"
source_url = "https://dl.google.com/android/repository/android-ndk-r29-linux.zip"
source_filename = "android-ndk-r29-linux.zip"
source_hash = "4abbbcdc842f3d4879206e9695d52709603e52dd68d3c1fff04b3b5e7a308ecf"

[[toolchain]]
name = "android_ndk_toolchain_aarch64"
wrap_name = "android_ndk"
ar = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-ar"
cc = "toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android35-clang"
cpp = "toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android35-clang++"
strip = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-strip"

[[toolchain]]
name = "android_ndk_toolchain_x86"
wrap_name = "android_ndk"
ar = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-ar"
cc = "toolchains/llvm/prebuilt/linux-x86_64/bin/i686-linux-android35-clang"
cpp = "toolchains/llvm/prebuilt/linux-x86_64/bin/i686-linux-android35-clang++"
strip = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-strip"

[[toolchain]]
name = "android_ndk_toolchain_x86_64"
wrap_name = "android_ndk"
ar = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-ar"
cc = "toolchains/llvm/prebuilt/linux-x86_64/bin/x86_64-linux-android35-clang"
cpp = "toolchains/llvm/prebuilt/linux-x86_64/bin/x86_64-linux-android35-clang++"
strip = "toolchains/llvm/prebuilt/linux-x86_64/bin/llvm-strip"

[[platform]]
name = "android_arm64"
toolchain = "android_ndk_toolchain_aarch64"

[platform.sysroot]
wrap_name = "android_ndk"
path = "toolchains/llvm/prebuilt/linux-x86_64/sysroot"