Avoid undefined behaviour in gas's rotate_left() macro for n == 0.

Otherwise, clang can effectively remove the first iteration of the for
loops where this macro is invoked, and as a result, "cmp r0, #99" fails
to assemble.

Obtained from:	joerg at netbsd
MFC after:	3 days

contrib/binutils/gas/config/tc-arm.c

@@ -6079,7 +6079,7 @@ parse_operands (char *str, const unsigned char *pattern)
 
 /* Functions for operand encoding.  ARM, then Thumb.  */
 
-#define rotate_left(v, n) (v << n | v >> (32 - n))
+#define rotate_left(v, n) (v << (n % 32) | v >> ((32 - n) % 32))
 
 /* If VAL can be encoded in the immediate field of an ARM instruction,
    return the encoded form.  Otherwise, return FAIL.  */