[compiler-rt] respect CMAKE_EXE_LINKER_FLAGS in compiler and library tests

Summary: check_cxx_compiler_flag and check_library_exists could fail because they ignored CMAKE_EXE_LINKER_FLAGS and therefore would always fail to produce executables. Cmake policy CMP0056 fixes this, but was explicitly set to OLD in our CMakeLists because it caused problems with test_target_arch. This change sets the policy to NEW to fix the problem with the compiler and library tests, and temporarily clears CMAKE_EXE_LINKER_FLAGS inside test_target_arch to emulate the old behavior there. This allows, for example, LTO builds that require lld to succeed.

Reviewers: davidxl, beanz

Reviewed By: beanz

Subscribers: fjricci, dberris, mgorny, mehdi_amini, tejohnson, rnk, llvm-commits

Differential Revision: https://reviews.llvm.org/D31098

llvm-svn: 298413