testsuite: RISC-V: Add effective target for E ABI variant

Add new effective target check for either ILP32E or ILP64E ABI variants.

Initial implementation only checks for RV32E or RV64E ISA, which in turn
implies that ILP32E/ILP64E ABI is used.  The RV32I+ILP32E and
RV64I+ILP64E combinations are not yet caught by the check, but they
do not seem to be widely used currently.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_riscv_abi_e):
	New procedure.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>