Improve IFUNC check [BZ #25506]

GNU ld's RISCV port does not support IFUNC. ld -no-pie produces no
relocation and the test passed incorrectly. Be more rigid by testing
IRELATIVE explicitly.

Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>