Strip libgcc to only keep fallback symbols

We use libgcc as fallback for symbols not present in libclang_rt
builtins, however we didn't know what exact symbols were being used,
some may not be intended to fallback.

Create libgcc_stripped, which only contains unwind symbols from libgcc.

Bug: 29275768
Test: bionic-unit-tests
Change-Id: I98df02ead7f6cca4e76ec92d4f880de4e03f5b5c
Merged-In: I5b349fa6138e51663bf3b67109b880b4356da8e8
(cherry picked from commit acee27cd)