[PR target/116662][RISC-V] Adjust destructive interference size for RISC-V

[PR target/116662][RISC-V] Adjust destructive interference size for RISC-V

So per the discussion in PR 116662, this adjusts the destructive interference
size for RISC-V to be more in line with current designs (64 bytes).

Getting this wrong is "just" a performance issue, so there's no correctness
concerns to be worried about.  The only real worry is that the value can have
ABI implications.  The position that Jason and others have taken is that while
it can be mis-used in a way that gets exposed as ABI, that's inherently unsafe
and we issue warning diagnostics for those cases.

So here's the change to bump it to 64 bytes.  Tested on rv32 and rv64 embedded
targets.  Bootstrap on the Pioneer & BPI is in flight and not due to land for
several hours.  Will push once pre-commit CI has done its thing (and the
Pioneer might have finished its cycle by then, which I'll check, obviously).

	PR target/116662
gcc/
	* config/riscv/riscv.cc (riscv_option_override): Override
	default value for destructive interference size.