contrib: dracut: Conditionalize copying of libgcc_s.so.1 to glibc only

The issue that this is designed to work around is only applicable to
glibc, since it's caused by glibc's pthread_cancel() implementation
using dlopen on libgcc_s.so.1 (and therefor not triggering dracut to
include it in the initramfs). This commit adds an extra condition to the
workaround that tests for glibc via "ldconfig -p | grep -qF 'libc.so.6'"
(which should only be present on glibc systems).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Violet Purcell <vimproved@inventati.org>
Closes #14992