Repack libgcc.a to only include required objects

Previous solution by using objcopy uses a quirky behaviour of the GNU
objcopy and there is no equivalent option in llvm-objcopy.

Instead of removing symbols, extract and repack libgcc to only include
required objects.

Bug: 142585047
Test: presubmit
Change-Id: I58af74c18838f797e481da38c3265f0624fddf99