[RISCV] Add UNIMP instruction (32- and 16-bit forms)

This patch adds support for UNIMP in both 32- and 16-bit forms. The 32-bit 
form can be seen as a variant of the ECALL/EBREAK/etc. family of instructions. 
The 16-bit form is just all zeroes, which isn't a valid RISC-V instruction, 
but still follows the 16-bit instruction form (i.e. bits 0-1 != 11).

Until recently unimp was undocumented and supported just by binutils, which 
printed unimp for either the 16 or 32-bit form. Both forms are now documented 
<https://github.com/riscv/riscv-asm-manual/pull/20> and binutils now supports 
c.unimp <https://sourceware.org/ml/binutils-cvs/2018-11/msg00179.html>.

Differential Revision: https://reviews.llvm.org/D54316
Patch by Luís Marques.

llvm-svn: 347988