Revert "Create libgcc_stripped with llvm-objcopy"

We rely on a weird, GNU specific behaviour of objcopy:

$ aarch64-linux-android-objcopy -w libgcc.a stripped.a --strip-unneeded-symbol=*
$ llvm-nm --defined-only stripped.a | grep multi
0000000000000000 T __multi3

$ aarch64-linux-android-objcopy -w libgcc.a stripped.a --strip-unneeded-symbol=* --keep-symbol=FOO
$ llvm-nm --defined-only stripped.a | grep multi

llvm-objcopy has a different, more consistent behaviour that does not
mark exported symbols as unneeded when "--keep-symbol" flag exists.
However this behaviour will leave most symbols not deleted in the
libgcc_stripped library.

Revert the change while I'm implementing an alternative solution.

Test: presubmit
Bug: 142585047
Change-Id: I175e811f8a1f6afc6558267fc54e159ad5e12acf