Enable building LLVM cross-compilers on Linux

[grendello]

Linux doesn't need to build MXE since the distributions we support at
this point have mingw packaged and we rely on the packages to provide
cross-compilation environment for Windows. Neither does Linux build
require the Mono Windows environments to be built.

Previously, when one enable the Windows LLVM build the Mono Windows
runtime was attempted to build as well. The attempt would fail since
some Windows-only libraries required by Mono/libmonodroid aren't available
as packages on Linux.

This commit disables building of Mono Windows runtimes while making it
possible to build the LLVM Windows cross compiler.

[jonpryor]

I don't understand why this line and the following two lines are required. If the @Condition fails, they'll default to the empty string/undefined anyway.

[grendello]

It's to reset them to a known state. Properties may be set in the environment or msbuild command line and in this case we don't want that to be possible

[jonpryor]

That won't override values set on the command-line. msbuild /p:_LlvmNeededWindows32=Garbage always takes precedence, and is immutable.

[grendello]

Huh, I didn't know that, oh well.

[jonpryor]

The macOS+xbuild PR Build hung on adb attach. :-(

12:05:43 		adb I 08-21 12:05:43 90886 15597543 adb_io.cpp:75] readx: fd=3 wanted=4
# ...and then i killed it...
13:31:17 		Tool /Users/builder/android-toolchain/sdk/platform-tools/adb execution finished.

It was stuck for nearly 90 minutes. :-(

[atsushieno]

We had some network issue that caused build failures (Slow network in Cambridge 1CC causing build failures)

[atsushieno]

build

[radekdoulik]

build

[grendello]

build