RISC-V: Fix -march option parsing when extension exists.

RISC-V: Fix -march option parsing when  extension exists.

This patch fixes -march option parsing when `p` extension exists,
e.g., -march=rv64imafdcp should produce

.attribute arch, "rv64i2p0_m2p0_a2p0_f2p0_d2p0_c2p0_p"

rather than

.attribute arch, "rv64i2p0_m2p0_a2p0_f2p0_d2p0_c_p"

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c
	(riscv_subset_list::parsing_subset_version): Fix -march option parsing
	when `p` extension exists.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/attribute-18.c: New test.