s390x: Capture grep output in static PIE check

The test is not a run-time check, so update the description.
Also use readelf -W for a more stable output format and fix
an LC_ALL typo.

This avoids garbled configure messages:

checking for s390-specific static PIE requirements (runtime check)...  0x0000000000000017 (JMPREL)             0x280
yes