Pass native build args from top-level scripts

[am11]

Add ability to pass --cross, --clangx.y, --gccx.y and --cmakeargs to native scripts via top-level build.sh. e.g.

./build.sh --subsetCategory coreclr --clang8 --cross --cmakeargs -DFeatureX=1 --cmakeargs Y='testing'
./build.sh --subsetCategory installer --gcc5 --cross --cmakeargs -DFeatureX=1 --cmakeargs Y='testing'
./build.sh --subsetCategory libraries --clang --cross --cmakeargs -DFeatureX=1 --cmakeargs Y='testing'

Also address CR feedback from PRs #1753 and #1602 (in separate commits).

[am11]

[IMO]
Now that all native build scripts share common arguments via:

runtime/eng/native/build-commons.sh

Lines 160 to 194 in e2325f3

	usage()
	{
	echo"Usage: $0 <options>"
	echo""
	echo"Common Options:"
	echo""
	echo"BuildArch can be: -arm, -armel, -arm64, -armel, x64, x86, -wasm"
	echo"BuildType can be: -debug, -checked, -release"
	echo"-bindir: output directory (defaults to $__ProjectRoot/artifacts)"
	echo"-ci: indicates if this is a CI build."
	echo"-clang: optional argument to build using clang in PATH (default)."
	echo"-clangx.y: optional argument to build using clang version x.y."
	echo"-cmakeargs: user-settable additional arguments passed to CMake."
	echo"-configureonly: do not perform any builds; just configure the build."
	echo"-cross: optional argument to signify cross compilation,"
	echo" will use ROOTFS_DIR environment variable if set."
	echo"-gcc: optional argument to build using gcc in PATH."
	echo"-gccx.y: optional argument to build using gcc version x.y."
	echo"-msbuildonunsupportedplatform: build managed binaries even if distro is not officially supported."
	echo"-ninja: target ninja instead of GNU make"
	echo"-numproc: set the number of build processes."
	echo"-portablebuild: pass -portablebuild=false to force a non-portable build."
	echo"-skipconfigure: skip build configuration."
	echo"-skipgenerateversion: disable version generation even if MSBuild is supported."
	echo"-stripsymbols: skip native image generation."
	echo"-verbose: optional argument to enable verbose build output."
	echo""
	echo"Additional Options:"
	echo""
	foriin"${!usage_list[@]}";do
	echo"${usage_list[${i}]}"
	done
	echo""
	exit 1
	}

we can extract these argument construction lines from coreclr/coreclr.proj, installer/corehost/build.proj and libraries/Native/build-native.proj into a common .props file, to avoid three different approaches of doing exactly the same thing (coreclr: ItemGroup, installer: property concatenation and libraries: separate properties followed by concatenation).

[janvorli]

cc: @jkoritzinsky, @ViktorHofer

[am11]

@janvorli, could you please give it a pass?

[janvorli]

LGTM, thank you!

[janvorli]

@am11 thank you for spending time to figure out a cleaner solution to the problem of passing the argument with spaces.