On RV64, long long (64-bit) fits in a single general-purpose register.
The existing load/store calls already handle 64-bit values correctly.
The 'not implemented' errors were vestigial from 32-bit architectures
where long long requires a register pair.
Removing these errors allows inline asm to accept 64-bit integer operands
on RV64, which is important since this is the native register width.
