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

cc/config/x86_linux_host.go

@@ -45,6 +45,7 @@ var (
 
 	linuxMuslCflags = []string{
 		"-D_LIBCPP_HAS_MUSL_LIBC",
+		"-DANDROID_HOST_MUSL",
 		"-nostdlibinc",
 	}