libgcc_s: add libgcc_s_asneeded.so wrapper for gcc 16

[siva]

Summary

GCC 16[0][1] now requires a wrapper lib to handle
'--push-state --as-needed -lgcc_s --pop-state' logic[1]
for gcc invocations. g++ will still unconditionally
use -lgcc_s.

This fixes buildworld with gcc16.

[0] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=8a99fdb70493df1294b53406913e5ea1fc971c13
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123396