[PR/target 102957] Allow Z*-ext extension with only 2 char.

[PR/target 102957] Allow Z*-ext extension with only 2 char.

We was assume the Z* extension should be more than 2 char, so we put an
assertion there, but it should just an error or warning rather than an
assertion, however RISC-V has add `Zk` extension, which just 2 char, so
actually, we should just allow that.

gcc/ChangeLog

	PR target/102957
	* common/config/riscv/riscv-common.c (multi_letter_subset_rank): Remove
	assertion for Z*-ext.

gcc/testsuite/ChangeLog

	* gcc.target/riscv/pr102957.c: New.