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>
diff --git a/src/cpu/x86/smm/smmhandler.S b/src/cpu/x86/smm/smmhandler.S
index 3750e52..b7805d0 100644
--- a/src/cpu/x86/smm/smmhandler.S
+++ b/src/cpu/x86/smm/smmhandler.S
@@ -43,7 +43,7 @@
 
 #define SMM_HANDLER_OFFSET 0x0000
 
-#if defined(__x86_64__)
+#if ENV_X86_64
 .bss
 ia32efer_backup_eax:
 .long 0
@@ -166,7 +166,7 @@
 	addl	$SMM_STACK_SIZE, %ebx
 	movl	%ebx, %esp
 
-#if defined(__x86_64__)
+#if ENV_X86_64
 	/* Backup IA32_EFER. Preserves ebx. */
 	movl	$(IA32_EFER), %ecx
 	rdmsr
@@ -180,7 +180,7 @@
 	/* Call C handler */
 	call	smi_handler
 
-#if defined(__x86_64__)
+#if ENV_X86_64
 	/*
 	 * The only reason to go back to protected mode is that RSM doesn't restore
 	 * MSR registers and MSR IA32_EFER was modified by entering long mode.