RISC-V: Re-enable -msave-restore for shared libraries.

RISC-V: Re-enable -msave-restore for shared libraries.

This re-enables -msave-restore for shared libraries, and uses the
t-slibgcc-libgcc file to get the save-restore routines included directly
in shared libraries so that we don't need to indirect through the PLT
to reach them, which doesn't work.

	gcc/
	* config/riscv/riscv.c (riscv_option_override): Revert 2019-08-30
	change.

	libgcc/
	* config.host (riscv*-*-linux*): Add t-slibgcc-libgcc to tmake_file.
	(riscv*-*-freebsd*): Likewise.

From-SVN: r275478