[CodeGen] Clang does not choose aapcs-vfp calling convention for ARM bare...

[CodeGen] Clang does not choose aapcs-vfp calling convention for ARM bare metal target with hard float (EABIHF)

Summary:
Clang does not detect `aapcs-vfp` for the EABIHF environment. The reason is that only GNUEABIHF is considered while choosing calling convention, EABIHF is ignored.

This causes clang to use `aapcs` for EABIHF and add the `arm_aapcscc` specifier to functions in generated IR.

The modified `arm-cc.c` test checks that no calling convention specifier is added to functions for EABIHF, which means the default one is used (`CallingConv::ARM_AAPCS_VFP`).

Reviewers: rengolin, compnerd, t.p.northover

Subscribers: aemerson, rengolin, asl, cfe-commits

Differential Revision: http://reviews.llvm.org/D20219

llvm-svn: 269419