RISC-V: tighten post-relocation-operator separator expectation

As per the spec merely a blank isn't okay as a separator, the operand
to the relocation function ought to be parenthesized. Enforcing this
then also eliminates an inconsistency in that

	lui	t0, %hi sym
	lui	t0, %hi 0x1000

were accepted, but

	lui	t0, %hi +sym
	lui	t0, %hi -0x1000

were not.

gas/config/tc-riscv.c

@@ -2200,7 +2200,9 @@ parse_relocation (char **str, bfd_reloc_code_real_type *reloc,
       {
 	size_t len = 1 + strlen (percent_op->str);
 
-	if (!ISSPACE ((*str)[len]) && (*str)[len] != '(')
+	while (ISSPACE ((*str)[len]))
+	  ++len;
+	if ((*str)[len] != '(')
 	  continue;
 
 	*str += len;

gas/testsuite/gas/riscv/auipc-parsing.l

@@ -3,3 +3,5 @@
 .*: Error: illegal operands `lui x10,x11'
 .*: Error: illegal operands `auipc x12,symbol'
 .*: Error: illegal operands `lui x13,symbol'
+.*: Error: illegal operands `auipc x14,%hi symbol'
+.*: Error: illegal operands `lui x15,%hi symbol'

gas/testsuite/gas/riscv/auipc-parsing.s

@@ -4,3 +4,6 @@
 # Don't accept a symbol without %hi() for 'u' operands.
 	auipc	x12,symbol
 	lui	x13,symbol
+# Don't accept reloc functions without parentheses.
+	auipc	x14,%hi symbol
+	lui	x15,%hi symbol