Make memmove an alias for memcpy

memcpy was an alias for bcopy with arg swap. This code handles
overlapping copies, so making memmove an alias is safe. We can
eliminate the call from libkern's memmove to this bcopy as a result.

Differential Revision: https://reviews.freebsd.org/D15374

sys/conf/files.mips

@@ -61,7 +61,6 @@ libkern/ffsll.c				standard
 libkern/fls.c				standard
 libkern/flsl.c				standard
 libkern/flsll.c				standard
-libkern/memmove.c			standard
 libkern/cmpdi2.c			optional	mips | mipshf | mipsel | mipselhf
 libkern/ucmpdi2.c			optional	mips | mipshf | mipsel | mipselhf
 libkern/ashldi3.c			standard

sys/mips/mips/bcopy.S

@@ -65,6 +65,7 @@ __FBSDID("$FreeBSD$");
 #define	SIZEREG	a2
 
 LEAF(memcpy)
+XLEAF(memmove)
 	.set	noat
 	.set	noreorder