RISC-V: Disable -msave-restore for shared libraries.

RISC-V: Disable -msave-restore for shared libraries.

This was noticed while trying to test -msave-restore support.  The
save/restore routines use the alternate return register t0/x5 which is
clobbered by the PLT header, so we can't use them in shared libraries.
This patch disables -msave-restore when -fpic (and -mplt), and emits a
warning if the user explicitly turned on -msave-restore.

	gcc/
	* config/riscv/riscv.c (riscv_option_override): If -msave-restore
	and -fpic and -mplt then disable -msave-restore and warn.

From-SVN: r275231