Move host flags from linux -> host

There will be a second linux host config (linux_bionic), so move this to
apply to all supported host configs instead of just linux_glibc.

Test: out/soong/build.ninja is the same before/after
Change-Id: Id8db10e0a3db889736070eea16f387785c913179

libunwindstack/Android.bp

@@ -67,7 +67,7 @@ cc_library {
 
     target: {
         // Always disable optimizations for host to make it easier to debug.
-        linux: {
+        host: {
             cflags: ["-O0", "-g"],
         },
     },