[test] #ifdef new builtin tests for __arm__ platform

Add an #if that excludes the newly added aeabi* tests on non-ARM
platforms. This is consistent with other ARM tests, and aims to make
running builtin tests easier. Lacking a proper infrastructure to run
tests selectively, it is more convenient if we do not have to implement
directory-platform exclusions and can just rely on tests compiling to
no-op on other platforms.

Differential Revision: https://reviews.llvm.org/D29708

llvm-svn: 294438