Define 'lr' as x30 on aarch64

GNU toolchain does not recognize LR as standard register alias,
but clang does. Use of #define will work on both. Place the
definition into central machine/asm.h instead of patching every
affected file, as requested by plaftorm maintainers.

Reviews by: andrew, emaste, imp
Differential Revision: https://reviews.freebsd.org/D10307