xen: arm: correct platform detection in public header.

These headers cannot use the CONFIG_FOO defines provided when building Xen
(since they aren't provided when building tools or by external components) and
need to use the compiler provided architecture defines.

This manifested itself as a failure to build xenctx.c on ARM64 due to the
missing symbols contains .

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

xen/include/public/arch-arm.h

@@ -218,7 +218,7 @@ typedef uint64_t xen_callback_t;
 #define PSR_MODE_SYS 0x1f
 
 /* 64 bit modes */
-#ifdef CONFIG_ARM_64
+#ifdef __aarch64__
 #define PSR_MODE_BIT  0x10 /* Set iff AArch32 */
 #define PSR_MODE_EL3h 0x0d
 #define PSR_MODE_EL3t 0x0c