Change TARGET_I386 to ARCH_X86

This renames TARGET_I386 to ARCH_X86 to make it more uniform with
other parts of the codebase, e.g. cbfs_core.h from cbfstool.

Change-Id: I1babcc941245ed1dde0478a21828766759373a42
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/1961
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Dave Frodin <dave.frodin@se-eng.com>

payloads/libpayload/libc/exec.c

@@ -30,7 +30,7 @@
 #include <libpayload-config.h>
 #include <libpayload.h>
 
-#ifdef CONFIG_TARGET_I386
+#ifdef CONFIG_ARCH_X86
 extern void i386_do_exec(long addr, int argc, char **argv, int *ret);
 #endif
 
@@ -46,7 +46,7 @@ int exec(long addr, int argc, char **argv)
 {
 	int val = -1;
 
-#ifdef CONFIG_TARGET_I386
+#ifdef CONFIG_ARCH_X86
 	i386_do_exec(addr, argc, argv, &val);
 #endif
 	return val;