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/arch/x86/cpu_common.c

@@ -2,7 +2,7 @@
 
 #include <cpu/cpu.h>
 
-#ifndef __x86_64__
+#if ENV_X86_32
 /* Standard macro to see if a specific flag is changeable */
 static inline int flag_is_changeable_p(uint32_t flag)
 {