RISC-V: Handle R_RISCV_CALL_PLT reloc

RISC-V: Handle R_RISCV_CALL_PLT reloc

GNU assembler starting 2.40 release always generates R_RISCV_CALL_PLT
reloc for call in assembler [1], similarly LLVM does not make
distinction between R_RISCV_CALL_PLT and R_RISCV_CALL [2].

Fixes "grub-mkimage: error: relocation 0x13 is not implemented yet.".

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=70f35d72ef04cd23771875c1661c9975044a749c
[2] https://reviews.llvm.org/D132530

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>