[tests] Add missing "int_lib.h" includes and extend guards

Fix missing "int_lib.h" includes in ARM negdf2vfp and subdf3vfp tests.
Additionally, extend the __arm__ guard to cover the builtin definition
in the former. This is mostly intended to prevent build failures
and allow those tests to be properly skipped on other targets.

Differential Revision: https://reviews.llvm.org/D28078

llvm-svn: 290422