Add comment about disabling crossgen command for cross build

Add comment about disabling crossgen command for cross build

As we all know, the current cross build generates exectuable
'crossgen' with arm architecture (without x86 architecture).
So, it can not be run to make mscorlib.ni.dll from mscorlib.dll
on X86-based build machine.
Let's add a comment why we have to skip crossgen command in
build_mscorlib() of build.sh in case of cross build for Linux/ARM.

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/af1b954466de2b931809434bf8ad4dc355dc3e98