[AArch64] Add feature flags and command line for ARMv8.2 FP16 support.

[Joseph Myers]

Out of interest, when will ARMv8.1 and v8.2 be documented in the ARM 
Architecture Reference Manual?  
<https://community.arm.com/groups/processors/blog/2014/12/02/the-armv8-a-architecture-and-its-ongoing-development> 
said "mid-2015" for v8.1, but as far as I can see DDI 0487A.h is current 
and doesn't seem to mention v8.1 anywhere obvious.

-- 
Joseph S. Myers
joseph@codesourcery.com