Turn _mcount() into a call to abort() on MIPS n64

When linking world with LLD it will complain about undefined references
to _mcount. For some reason the _mcount function definition will end
up in the .mdebug.abi section and therefore the linker won't find it.

This change should be fine because the inline assembly below won't work
for N64 anyway.

sys/mips/include/profile.h

@@ -42,7 +42,10 @@
 /*XXX The cprestore instruction is a "dummy" to shut up as(1). */
 
 /*XXX This is not MIPS64 safe. */
-
+#ifdef __mips_n64
+/* XXXAR: not implemented for mips n64 */
+#define	MCOUNT	void _mcount(uintfptr_t frompc, uintfptr_t selfpc) { abort(); }
+#else
 #define	MCOUNT \
 	__asm(".globl _mcount;"		\
 	".type _mcount,@function;"	\
@@ -71,7 +74,7 @@
 	"move $31,$1;"			\
 	".set reorder;"			\
 	".set at");
-
+#endif
 #ifdef _KERNEL
 /*
  * The following two macros do splhigh and splx respectively.