CodeGen: ensure that libcalls are always AAPCS CC

All of the builtins are designed to be invoked with ARM AAPCS CC even on ARM
AAPCS VFP CC hosts.  Tweak the default initialisation to ARM AAPCS CC rather
than C CC for ARM/thumb targets.

The changes to the tests are necessary to ensure that the calling convention for
the lowered library calls are honoured.  Furthermore, these adjustments cause
certain branch invocations to change to branch-and-link since the returned value
needs to be moved across registers (d0 -> r0, r1).

llvm-svn: 280683