Allow -march=armv7a on the gcc command line, for compatibility with clang.
This will result in __ARM_ARCH_7A__ being defined during the compile.

When compiling with gcc, it will still only generate armv6 opcodes itself,
but should pass the arch to gas so that inline asm can use v7 opcodes.
Add __FreeBSD_version guards around hsearch_r to ease MFCing the code to
stable/10

It was added when __FreeBSD_version was ~1100027
Add -march=armv7a to the kernel compile for all ARM systems which are v7a.

Submitted by:	Michal Meloun <meloun@miracle.cz>