sys-libs/glibc: Reconcile divergences in stripping logic

Remove the downstream-specific line "export PORTAGE_STRIP_FLAGS="--strip-debug -N gentoo_check_ldflags -R .comment""

The issue mentioned in the large comment points to https://sourceware.org/gdb/wiki/FAQ as a justification for adding the line. However it is clear from there that only libpthread needs to keep its symbol table, and the export above is applying the new stripping logic to all files. Moreover, this line is redundant, since we are already keeping the symbol table for libpthread in "dostrip -x (alt_libdir)/libpthread-{upstream_pv}.so"

which is the upstream solution to the potential problem.

Also, add the big the big wall comment from Gentoo upstream describing the stripping logic applied to the ebuild.

With these changes, we reconcile with upstream the differences in the stripping-related code.