Windows arm64 build: any -cmakeargs breaks the x64 cross-components build #135468

On a Windows arm64 build, passing any -cmakeargs breaks the x64 cross-components build:

build.cmd clr.runtime -a arm64 -c Debug -cmakeargs -DCLR_PROBE_NOOP=1

artifacts\obj\coreclr\windows.arm64.Debug\x64\debug\di\floatconversion.asm(2748) : error A2008:syntax error : GBLA

The same command without -cmakeargs builds. CLR_PROBE_NOOP is a name nothing reads; -DFEATURE_CORPROFILER=0 fails the same way.

After the failure, x64\CMakeCache.txt has one fused entry, CLR_PROBE_NOOP:UNINITIALIZED=1 -DCLR_CROSS_COMPONENTS_BUILD=1, and no CLR_CROSS_COMPONENTS_BUILD entry of its own. So the cross-components sub-build configures as a full arm64 build and assembles arm64 sources with ml64.

Cause: eng/Subsets.props (lines 421, 462, 475) appends the flag with a space, CMakeArgs=$(CMakeArgs) -DCLR_CROSS_COMPONENTS_BUILD=1, and src/coreclr/runtime.proj (lines 25-26) turns each _CMakeArgs item into one quoted -cmakeargs "...". With an empty CMakeArgs the leading space is harmless; with a user value, both definitions end up in a single argument.

Splitting in runtime.proj with $([MSBuild]::Unescape($(CMakeArgs.Replace(' ', ';')))) makes the build pass (both definitions then appear as separate cache entries), but it would break a CMake argument that contains a space, so it is a workaround rather than a fix.

Reproduced on a fresh obj dir at ef6ca45 (#135377's head); both files are identical on main (98a24e3).