More ARM EABI object attributes in binutils. This adds support to binutils
to include the Unaligned Access and Floating-point Half-precision
attributes. the former marks ELF objects that may access ARMv6 style
unaligned data, the latter that the binary uses the VFPv3/Advanced SIMD
half-precision extension.

These may be emmitted by clang so it's best to print a warning when the
linker hits one of them.

Differential Revision: http://reviews.freebsd.org/D2194
Submitted by: Michal Meloun <meloun@miracle.cz>
MFC after: 1 week