Move MI-ish bcopy routine to libkern

riscv and powerpc have nearly identical bcopy.c that's
supposed to be mostly MI. Move it to the MI libkern.

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

sys/conf/files.powerpc

@@ -85,6 +85,7 @@ kern/subr_sfbuf.c		standard
 libkern/ashldi3.c		optional	powerpc | powerpcspe
 libkern/ashrdi3.c		optional	powerpc | powerpcspe
 libkern/bcmp.c			standard
+libkern/bcopy.c			standard
 libkern/cmpdi2.c		optional	powerpc | powerpcspe
 libkern/divdi3.c		optional	powerpc | powerpcspe
 libkern/ffs.c			standard
@@ -195,7 +196,6 @@ powerpc/powernv/powernv_centaur.c	optional	powernv
 powerpc/powernv/powernv_xscom.c	optional	powernv
 powerpc/powerpc/altivec.c	optional	powerpc | powerpc64
 powerpc/powerpc/autoconf.c	standard
-powerpc/powerpc/bcopy.c		standard
 powerpc/powerpc/bus_machdep.c	standard
 powerpc/powerpc/busdma_machdep.c standard
 powerpc/powerpc/clock.c		standard

sys/conf/files.riscv

@@ -12,6 +12,7 @@ kern/kern_clocksource.c		standard
 kern/subr_devmap.c		standard
 kern/subr_dummy_vdso_tc.c	standard
 libkern/bcmp.c			standard
+libkern/bcopy.c			standard
 libkern/ffs.c			standard
 libkern/ffsl.c			standard
 libkern/ffsll.c			standard
@@ -21,7 +22,6 @@ libkern/flsll.c			standard
 libkern/memmove.c		standard
 libkern/memset.c		standard
 riscv/riscv/autoconf.c		standard
-riscv/riscv/bcopy.c		standard
 riscv/riscv/bus_machdep.c	standard
 riscv/riscv/bus_space_asm.S	standard
 riscv/riscv/busdma_machdep.c	standard