[Clang][Arm] Fix fp16 return error tests under AArch64/Arm. NFC

The -fallow-half-arguments-and-returns option was removed in
59528e4bdb27ed4ab3, replaced with an always-on target option under
AArch64/Arm. There are two tests - fp16-sema.c and renderscripts.rs that
test that an error is produced for __fp16 function args/returns, which
are now expected to pass for Arm/AArch64. i.e they no longer give the
same error as before on native Arm/AArch64 machines. Alter the targets
of those tests to compensate.