llvm: [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.

Obtained from: llvm r338370 by Peter Smith