Set ANDROID_HOST_MUSL for musl builds

Unlike glibc, musl does not #define __MUSL__ on the assumption that
building against musl will be done using a configure script that should
be testing for individual features rather than assuming them based on
using musl.  We don't use configure-based builds, so add a local
ANDROID_HOST_MUSL macro that will be defined for all host musl builds.

Bug: 190084016
Test: m USE_HOST_MUSL=true adb
Change-Id: I44e3ffd1d727fd0ea8b69c3b074fef7e92e2be41