[RISCV] Match GNU tools canonical JALR and add aliases

The canonical GNU form of JALR resembles a load/store instruction rather
than placing the immediate offset as a separate argument, so match this
behaviour. Also add parser-only aliases for the three-operand form, and
add other shorter aliases also emitted by GNU tools.

Differential Revision: https://reviews.llvm.org/D55277
Patch by James Clarke.

llvm-svn: 366179