x86: seperate memtest from init_64.c

it's separate functionality that deserves its own file.

This also prepares 32-bit memtest support.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index 9873716..1fbb844 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -21,3 +21,4 @@
 endif
 obj-$(CONFIG_ACPI_NUMA)		+= srat_$(BITS).o
 
+obj-$(CONFIG_MEMTEST)		+= memtest.o