ARM: Remove check for isAAPCS_ABI when enabling various aeabi calls
Based on computeDefaultTargetABI, this appears to be redundant.
The only test using the explicit -target-abi flag with a value different
than one implied by the triple (i.e. assert(TM.isAAPCS_ABI()) under
the condition) is arm-abi-attr.ll and it doesn't stress the libcalls
used.