RISC-V: Always pass -misa-spec to assembler [PR104219]

RISC-V: Always pass -misa-spec to assembler [PR104219]

Add -misa-spec to OPTION_DEFAULT_SPECS to make sure -misa-spec will
always pass that into assembler, that prevent GCC and binutils using
different way to interpret the ISA string.

gcc/ChangeLog:

	PR target/104219
	* config.gcc (riscv*-*-*): Normalize the with_isa_spec value.
	(all_defaults): Add isa_spec.
	* config/riscv/riscv.h (OPTION_DEFAULT_SPECS): Add isa_spec.