[Android] Default to --rtlib=compiler-rt

[rprichard]

Summary

By default, the driver uses the compiler-rt builtins and links with
-l:libunwind.a.

Restore the previous behavior by passing --rtlib=libgcc.

[rprichard]

Adding glandium and thakis for Firefox and Chrome. e.g. I suspect this change would cause the same sort of breakage seen in D95166.

[rprichard]

Rebase this revision.

[smeenai]

Hilariously enough, this breaks building compiler-rt itself inside LLVM's runtime builds setup for us. The runtimes build setup builds clang and then uses the just-built clang to build compiler-rt. That build fails to link since my just-built clang doesn't have compiler-rt available, because it's currently trying to build compiler-rt itself. That's a bug in the compiler-rt build system, and I sent out https://lists.llvm.org/pipermail/llvm-dev/2021-March/149137.html to ask what we should do about it.