RISC-V: make C-extension JAL available again for (32-bit) assembly

Along with the normal JAL alias, the C-extension one should have been
moved as well by 839189bc ("RISC-V: re-arrange opcode table for
consistent alias handling"), for the assembler to actually be able to
use it where/when possible.

Since neither this nor any other compressed branch insn was being tested
so far, take the opportunity and introduce a new testcase covering those.