RISC-V: Remove RV64E conflict

Since RV32E *and* RV64E are ratified, RV64E is no longer invalid.

This commit removes a restriction that prevents making base ISA with
reduced GPRs with XLEN > 32.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_check_conflicts): Remove RV64E
	conflict since the ratified 'E' base ISAs include RV64E.

gas/ChangeLog:

	* testsuite/gas/riscv/march-fail-base-02.d: Removed.
	* testsuite/gas/riscv/march-fail-base-02.l: Removed.