[gn build] Fix Android compiler-rt targets

If a sysroot was specified, it would take precedence over the Android
NDK sysroot since it would appear after in the command line.

Also only build runtimes for enabled target arches. Many places have
copied this around so create and use supported_android_toolchains.

Reviewed By: pcc

Differential Revision: https://reviews.llvm.org/D113606