[RISCV] Reduce size of CSR lookup tables. NFC (#121606)
Instead of storing 3 different names in each row of the table, use a
separate row for each name and use a flag to indicate what type of name
it is. The AltName and DeprecatedName weren't used often enough to
justify storing them as a possibility for every register.
This reduces the .rodata size by 27k and reduces the number of dynamic
relocations since we now only need 1 lookup by name function. The lookup
by name function each contained a ~400 entry table of const char*
pointing to constant strings. Each of those requires a dynamic
relocation.
I also capitalized IsRV32Only in the C++ code to match coding
standards.
