GCC supports a small number of RISC-V specific inline assembly
constraints, while currently clang only supports the machine independent
ones for RISC-V targets. Add parsing for these constraints, based on
their descriptions in the GCC documentation .
Apologies if I didn't tag the right reviewers; I took a look at those who seemed to be most involved with the RISC-V target, but if there's anyone more appropriate to review clang changes then feel free to add them instead :)