x86/public: correct register naming

Commit 897129deab ("x86: use unambiguous register names") went a little
too far: With it we also get register names like _e15 and e15 for
non-Xen consumers using a gcc compatible compiler. Correct this.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/public/arch-x86/xen-x86_64.h
+++ b/xen/include/public/arch-x86/xen-x86_64.h
@@ -173,7 +173,7 @@ struct iret_context {
 #define __DECL_REG_LOHI(name) __DECL_REG(name ## x)
 #define __DECL_REG_LO8        __DECL_REG
 #define __DECL_REG_LO16       __DECL_REG
-#define __DECL_REG_HI         __DECL_REG
+#define __DECL_REG_HI(num)    uint64_t r ## num
 #endif
 
 struct cpu_user_regs {