[RISCV] Mark C.EBREAK instruction as having side effects

C.EBREAK was defined with hasSideEffects = 0, which is incorrect and 
inconsistent with the non-compressed instruction form. This patch corrects 
this oversight.

This wouldn't cause codegen issues, as compressed instructions are only ever 
generated by converting the non-compressed form as an MCInst. But having 
correct flags is still worthwhile.

Differential Revision: https://reviews.llvm.org/D54256
Patch by Luís Marques.

llvm-svn: 346959