arch/x86: Use ENV_X86_64 instead of _x86_64_

Tested on Intel Sandybridge x86_64 and x86_32.

Change-Id: I152483d24af0512c0ee4fbbe8931b7312e487ac6
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44867
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/soc/intel/denverton_ns/soc_util.c

@@ -274,7 +274,7 @@ void *memcpy_s(void *dest, const void *src, size_t n)
 	unsigned long d0, d1, d2;
 
 	asm volatile(
-#ifdef __x86_64__
+#if ENV_X86_64
 		"rep ; movsd\n\t"
 		"mov %4,%%rcx\n\t"
 #else