sys-libs/glibc: restore strip quirk for 'libpthread.so.0'

sys-libs/glibc: restore strip quirk for 'libpthread.so.0'

This change effectively reverts commit e14229b10b513a
("sys-libs/glibc: drop prepallstrip, bug #587296")

gdb still relies on libpthread being unstripped.
See https://bugs.gentoo.org/697910 for details and reproducer.

Also see ebuild note that explains indirect mechanics
of how glibc relies on gdb's ability to traverse local symbols.

In EAPI=7 ebuilds (glibc-9999) we do selective stripping.

Reported-by: Minux
Closes: https://bugs.gentoo.org/697910
Bug: https://bugs.gentoo.org/587296
Bug: https://bugs.gentoo.org/46186
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>