Add support for Linux host

[MichalStrehovsky]

We're currently limited to Windows host because I was lazy to write the clang shim batch file in Bash.

PublishAotCross/src/clang.cmd

Lines 1 to 26 in 8fbc8fa

	@echo off
	@where zig >nul 2>&1
	@if ERRORLEVEL 1 (
	echo Error: zig is not on the PATH. Install zig and make sure it's on PATH. Follow instructions at https://github.com/MichalStrehovsky/PublishAotCross.
	exit /B 1
	)
	set args=%*
	rem Works around zlib not being available with zig. This is not great.
	set args=%args:-lz =%
	rem Work around a .NET 8 Preview 6 issue
	set args=%args:'-Wl,-rpath,$ORIGIN'=-Wl,-rpath,$ORIGIN%
	rem Work around parameters unsupported by zig. Just drop them from the command line.
	set args=%args:--discard-all=--as-needed%
	set args=%args:-Wl,-pie =%
	set args=%args:-pie =%
	rem Works around zig linker dropping necessary parts of the executable.
	set args=-Wl,-u,__Module %args%
	rem Run zig cc
	zig cc %args%

Windows host is also guarded here:

PublishAotCross/src/PublishAotCross.targets

Line 4 in 8fbc8fa

<Import Project="Crosscompile.targets" Condition="$(RuntimeIdentifier.Contains('linux')) and $([MSBuild]::IsOSPlatform('Windows'))" />

[scgm0]

I managed to get Linux to Windows cross-compilation working via msvc-wine; may I file a PR for that?
demo: https://github.com/scgm0/NativeAotTest