RISC-V/libgcc: Reduce the size of RV64 millicode by 6 bytes

RISC-V/libgcc: Reduce the size of RV64 millicode by 6 bytes

Rewrite code sequences throughout the 64-bit RISC-V `__riscv_save_*'
routines replacing `li t1, -48', `li t1, -64', and `li t1, -80',
instructions, which do not have a compressed encoding, respectively with
`li t1, 3', `li t1, 4', and `li t1, 4', which do, and then adjusting the
remaining code accordingly observing that `sub sp, sp, t1' takes the
same amount of space as an `slli t1, t1, 4'/`add sp, sp, t1' instruction
pair does, again due to the use of compressed encodings, saving 6 bytes
total.

This change does increase code size by 4 bytes for RISC-V processors
lacking the compressed instruction set, however their users couldn't
care about the code size or they would have chosen an implementation
that does have the compressed instructions, wouldn't they?

	libgcc/
	* config/riscv/save-restore.S [__riscv_xlen == 64]
	(__riscv_save_10, __riscv_save_8, __riscv_save_6, __riscv_save_4)
	(__riscv_save_2): Replace negative immediates used for the final
	stack pointer adjustment with positive ones, right-shifted by 4.