ELF: Handle R_ARM_V4BX correctly in PIC output files.

Previously we considered R_ARM_V4BX to be an absolute relocation,
which meant that we rejected it in read-only sections in PIC output
files. Instead, treat it as a hint relocation so that relocation
processing ignores it entirely.

Also fix a problem with the test case where it was never being run
because it has a .yaml extension and we don't run tests with that
extension.

Differential Revision: https://reviews.llvm.org/D55728

llvm-svn: 349216