powerpc/code-patching: Move code patching selftests in its own file

Code patching selftests are half of code-patching.c.
As they are guarded by CONFIG_CODE_PATCHING_SELFTESTS,
they'd be better in their own file.

Also add a missing __init for instr_is_branch_to_addr()
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c0c30504f04eb546a48ff77127a8bccd12a3d809.1638446239.git.christophe.leroy@csgroup.eu

arch/powerpc/lib/Makefile

@@ -21,6 +21,8 @@ endif
 
 obj-y += alloc.o code-patching.o feature-fixups.o pmem.o
 
+obj-$(CONFIG_CODE_PATCHING_SELFTEST) += test-code-patching.o
+
 ifndef CONFIG_KASAN
 obj-y	+=	string.o memcmp_$(BITS).o
 obj-$(CONFIG_PPC32)	+= strlen_32.o