RISC-V: Properly parse the letter 'p' in '-march'.

gcc/ChangeLog:
	* common/config/riscv/riscv-common.c
	(riscv_subset_list::parsing_subset_version): Properly parse the letter
	'p' in '-march'.
	(riscv_subset_list::parse_std_ext,
	 riscv_subset_list::parse_multiletter_ext): To handle errors generated
	in riscv_subset_list::parsing_subset_version.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/arch-12.c: New.
	* gcc.target/riscv/attribute-19.c: New.