RISC-V: PR30449, Add lga assembler macro support.

RISC-V: PR30449, Add lga assembler macro support.

Originally discussion, https://github.com/riscv/riscv-isa-manual/pull/539

Added new load address pseudo instruction which is always expanded to GOT
access, no matter the .option rvc is set or not.

gas/
	PR 30449
	* config/tc-riscv.c (macro): Add M_LGA support.
	* testsuite/gas/riscv/la-variants.d: New.
	* testsuite/gas/riscv/la-variants.s: New.
include/
	PR 30449
	* opcode/riscv.h (M_LGA): New.
opcodes/
	PR 30449
	* riscv-opc.c (riscv_opcodes): Add lga support.