RISC-V: Add "lp64e" ABI support

Since RV32E and RV64E are now ratified, this commit prepares the ABI
support for LP64E (LP64 with reduced GPRs).

gas/ChangeLog:

	* config/tc-riscv.c (riscv_set_abi_by_arch): Update the error
	message.  (md_parse_option): Accept "lp64e".
	* doc/c-riscv.texi: Update the documentation to allow "lp64e".
	* testsuite/gas/riscv/mabi-fail-rv32e-lp64f.l:
	Change error message.
	* testsuite/gas/riscv/mabi-fail-rv32e-lp64d.l: Likewise.
	* testsuite/gas/riscv/mabi-fail-rv32e-lp64q.l: Likewise.