Auto merge of #117504 - pcc:android-link-libunwind, r=Mark-Simulacrum

Remove obsolete support for linking unwinder on Android

Linking libgcc is no longer supported (see #103673), so remove the related link attributes and the check in unwind's build.rs. The check was the last remaining significant piece of logic in build.rs, so remove build.rs as well.

Author: bors