RISC-V: Support -mlittle-endian and -mbig-endian

RISC-V: Support -mlittle-endian and -mbig-endian

gcc/
	* config/riscv/elf.h (LINK_SPEC): Pass linker endianness flag.
	* config/riscv/freebsd.h (LINK_SPEC): Likewise.
	* config/riscv/linux.h (LINK_SPEC): Likewise.
	* config/riscv/riscv.h (ASM_SPEC): Pass -mbig-endian and
	-mlittle-endian.
	(BYTES_BIG_ENDIAN): Handle big endian.
	(WORDS_BIG_ENDIAN): Define to BYTES_BIG_ENDIAN.
	* config/riscv/riscv.opt (-mbig-endian, -mlittle-endian): New
	options.
	* doc/invoke.texi (-mbig-endian, -mlittle-endian): Document.