RISC-V: Fix wrong version number when arch contains 'p'.

When specify a default version for p extension in
riscv_supported_std_ext[](elfxx-riscv.c) and assembling with
-march=rv32imacp, the c extension's version in attribute will become
0p0, the expectation is 2p0.

TODO: Remember to add testcase when we have supported standrad p in
the future.

bfd/
	PR gas/28372
	* elfxx-riscv.c (riscv_parsing_subset_version): Break if p
	represent the 'p' extension.

Change-Id: Ia4e0cf26f3d7d07acaee8cefd86707ecac663a59