RISC-V: allow -fno-plt to disable PLT

Currently, the RISC-V target uses the target specific mplt option to
control PLT generation. This patch deprecates the target specific mplt
option and uses the common fplt option instead. This allows users to
use the same option for most targets.

Co-Developed-by: Liao Shihua <shihua@iscas.ac.cn>
Signed-off-by: Yangyu Chen <cyy@cyyself.name>

gcc/ChangeLog:

	* config/riscv/predicates.md: Use flag_plt instead of TARGET_PLT.
	* config/riscv/riscv.opt: alias common option fplt to mplt.