[ARM] Honor -mfloat-abi for libcall calling convention

As far as I can tell, this matches gcc: -mfloat-abi determines the
calling convention for all functions except those explicitly defined as
soft-float in the ARM RTABI.

This change only affects cases where the user specifies -mfloat-abi to
override the default calling convention derived from the target triple.

Fixes https://bugs.llvm.org//show_bug.cgi?id=34530.

Differential Revision: https://reviews.llvm.org/D38299

llvm-svn: 316708