[Builtin] Unxfail tests for armhf

Summary:
Originally, a few tests fail for armhf target due to:
1) COMPILER_RT_ARMHF_TARGET was not set when building the lib
2) COMPILER_RT_ABI should not be defined as `__attribute__((pcs("aapcs")))` for armhf when building for both lib and tests

This address https://bugs.llvm.org//show_bug.cgi?id=32261

mulsc3_test.c is a newly exposed issue, which will be addressed separately.

Reviewers: rengolin, compnerd

Reviewed By: compnerd

Subscribers: aemerson, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D31448

llvm-svn: 298974