Clean up darwin cc toolchain

Remove the ability for Soong to build 32-bit darwin code. We've already
disabled this in Make, this is just removing the unused bits in Soong
and simplifing the toolchain config.

Test: m host
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I968c5d98bdf162297d639f7988918dadb7ba6e07