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

sys/arm64/include/asm.h

@@ -57,6 +57,9 @@
 #define	PIC_SYM(x,y)	x
 #endif
 
+/* Alias for link register x30 */
+#define	lr		x30
+
 /*
  * Sets the trap fault handler. The exception handler will return to the
  * address in the handler register on a data abort or the xzr register to