Fix android-on-mac build

One of the last refactorings was causing gold to be picked as
host linker when building for target_os=android on Mac.
The previous code had a bunch of bugs:
1. Ignored the fact that is_android is still true in the
   toolchain definition file on mac when targeting android.
2. Was passing the wrong "linker" variable to the -fuse-ld
   argument, instead of the invoker one.

Test: manual (builds on mac)
Change-Id: Ibe3eee5ce9ef9ae960c65ad2c0b312b50e1d0041