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/include/asm-x86/e820.h b/include/asm-x86/e820.h
index 06633b0..16a31e2 100644
--- a/include/asm-x86/e820.h
+++ b/include/asm-x86/e820.h
@@ -90,6 +90,14 @@
 }
 #endif
 
+#ifdef CONFIG_MEMTEST
+extern void early_memtest(unsigned long start, unsigned long end);
+#else
+static inline void early_memtest(unsigned long start, unsigned long end)
+{
+}
+#endif
+
 extern unsigned long end_user_pfn;
 
 extern u64 find_e820_area(u64 start, u64 end, u64 size, u64 align);