Disable REGISTER_PREFIX_OPTIONAL

This is the default behaviour on standard ELF targets. It is necessary
to avoid clashes between register names and C variable/function names
when the leading underscore isn't used. So inside assembler sources, all
register names must be prefixed by the % character. And inside C
sources, in inline assembly blocks, the prefix must be escaped to %%.

It is still possible to avoid the % prefix requirement by adding
--register-prefix-optional to the as command line.

gas/config/te-mintelf.h

@@ -19,7 +19,6 @@
 
 #define TE_MINTELF
 #define LOCAL_LABELS_FB 1
-#define REGISTER_PREFIX_OPTIONAL 1
 
 /* The .lcomm directive mustn't try to align more than possible.  */
 #define TC_IMPLICIT_LCOMM_ALIGNMENT(SIZE, P2VAR)		\