lld: [ELF][ARM] Implement support for Tag_ABI_VFP_args

lld: [ELF][ARM] Implement support for Tag_ABI_VFP_args

The Tag_ABI_VFP_args build attribute controls the procedure call
standard used for floating point parameters on ARM. The values are:

0 - Base AAPCS (FP Parameters passed in Core (Integer) registers
1 - VFP AAPCS (FP Parameters passed in FP registers)
2 - Toolchain specific (Neither Base or VFP)
3 - Compatible with all (No use of floating point parameters)

If the Tag_ABI_VFP_args build attribute is missing it has an implicit
value of 0.

We use the attribute in two ways:

* Detect a clash in calling convention between Base, VFP and Toolchain.

we follow ld.bfd's lead and do not error if there is a clash between an
implicit Base AAPCS caused by a missing attribute. Many projects
including the hard-float (VFP AAPCS) version of glibc contain assembler
files that do not use floating point but do not have Tag_ABI_VFP_args.

* Set the EF_ARM_ABI_FLOAT_SOFT or EF_ARM_ABI_FLOAT_HARD ELF header flag

for Base or VFP AAPCS respectively. This flag is used by some ELF
loaders.

References:
* Addenda to, and Errata in, the ABI for the ARM Architecture for
  Tag_ABI_VFP_args
* Elf for the ARM Architecture for ELF header flags

Fixes LLVM PR36009

PR:		229050
Obtained from:	llvm r338377 by Peter Smith