Limit calls to strip.sh on darwin

strip.sh can use a file descriptor per .o file when run on .a files,
which can hit the system file descriptor limit on darwin.  This
causes failures when manay variants of libgcc_stripped are built
simultaneously.  Put all strip rules on darwin into a pool that
limits them to 10 concurrent processes, which will limit the file
descriptor usage to ~7500.

Fixes: 132822437
Test: no mention of darwinStripPool in out/soong/build.ninja on linux
Test: m libgcc_stripped on darwin
Change-Id: I3d4fbbd8d44d2e9059a79df113ab95336ec2c658