[ELF][ARM] Add Arm ABI names for float ABI ELF Header flags

The ELF for the Arm architecture document defines, for EF_ARM_EABI_VER5 and
above, the flags EF_ARM_ABI_FLOAT_HARD and EF_ARM_ABI_FLOAT_SOFT. These
have been defined to be compatible with the existing EF_ARM_VFP_FLOAT and
EF_ARM_SOFT_FLOAT used by gcc for EF_ARM_EABI_UNKNOWN.

This patch adds the flags in addition to the existing ones so that any code
depending on the old names will still work.

Differential Revision: https://reviews.llvm.org/D49992

llvm-svn: 338370