[PATCH] ARM build attributes support

[Tim Northover]

Hi,

> Can you just get someone at ARM to double check the attributes themselves?

I've compared it against the latest specification at
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0045d/IHI0045D_ABI_addenda.pdf,
and have a few minor points about the attributes (mostly from
ELF_ARM.h)

+ ABI_align8_needed has been renamed to ABI_align_needed (and same for
_preserved).
+ There are now v8 attributes in TAG_CPU_arch (14) and TAG_FP_arch (7 & 8)
+ A comment at the start of the TAG_thumb_arch enum is missing (to say
what AllowThumb2 applies to)
+ Tag_ABI_HardFP_use items look dodgy to me.
+ Tag_ABI_VFP_args has a new "3" entry.
+ The llvm-readobj code doesn't handle CPU_raw_name (I think).
+ Could also be worth adding a reference to the ABI document in
docs/CompilerWriterInfo.rst
+ A couple of enums seem to be missing, but I think that's a smaller
issue than partial support, so I'm happy to leave that.

Cheers.

Tim.