RISC-V: Always generate R_RISCV_CALL_PLT reloc for call in assembler.

Since we have the same behaviors of CALL and CALL_PLT relocs in linker for now,
https://github.com/bminor/binutils-gdb/commit/3b1450b38c644f99aa2e211747b428b9f8d15cca

And the psabi already deprecate the CALL reloc,
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/a0dced85018d7a0ec17023c9389cbd70b1dbc1b0

Therefore, we should always generate R_RISCV_CALL_PLT reloc for call, even if
it has @plt postfix.  I believe LLVM (https://reviews.llvm.org/D132530) already
support this, so GNU as should do the same thing.

gas/
	* config/tc-riscv.c (riscv_ip): Always generate CALL_PLT reloc for
	call, even if it has @plt postfix.
	* testsuite/gas/riscv/no-relax-reloc.d: Updated CALL to CALL_PLT.
	* testsuite/gas/riscv/relax-reloc.d: Likewise.
ld/
	* testsuite/ld-riscv-elf/variant_cc-r.d: Updated CALL to CALL_PLT.