riscv_register_name returns NULL to indicate there is no name, so check
for that.

Every CSR is accessible as a named register, csrNN. 'info registers all'
and 'info registers system' only show CSRs that have aliases defined.

This change renumbers the CSR register numbers used when talking to a
remote client, breaking CSR accessibility for existing RISC-V
gdbserver/stub implementations.

This is avoids code duplication, but is inefficient because
riscv_register_reggroup_p() is O(n). The right fix is to fix that
function.

Previously eg. "print $x1" wouldn't work but "print $ra" would. Now they
both work. When displaying registers the alias is still preferred.

Cleaned up a few RISCV_*_REGNUM definitions.
Changed whitespace to match existing code style.