[builtins][ARM] re-enable mulsc3_test.c on Arm

    
The mulsc3_test.c was marked as unsupported due to PR32457, the underlying
cause of this PR was fixed in PR28164 so we can remove the unsupported as
it is no longer needed.

Differential Revision: https://reviews.llvm.org/D40076

llvm-svn: 318396