[RISCV] Allow call pseudoinstruction to be used to call a function name that...

[RISCV] Allow call pseudoinstruction to be used to call a function name that coincides with a register name

Previously `call zero`, `call f0` etc would fail. This leads to compilation 
failures if building programs that define functions with those names and using 
-save-temps.

llvm-svn: 330846