RISC-V: Don't allow any uppercase letter in the arch string.

RISC-V: Don't allow any uppercase letter in the arch string.

Although I cannot find any RISC-V specs said that uppercases are not
allowed in the arhc string, but seems like it is an established fact
both for GNU and LLVM.  Therefore, we shouldn't allow the uppercases
for the non-standard x extensions, too.

	bfd/
	* elfxx-riscv.c (riscv_parse_subset): ISA string cannot contain
	any uppercase letter.

	gas/
	* testsuite/gas/riscv/march-fail-uppercase-base.d: Updated.
	* testsuite/gas/riscv/march-fail-uppercase.l: Updated.
	* testsuite/gas/riscv/march-fail-uppercase-x.d: New testcase.