Remove callback from cosmoaudio API

Using callbacks is still problematic with cosmo_dlopen() due to the need
to restore the TLS register. So using callbacks is even more strict than
using signal handlers. We are better off introducing a cosmoaudio_poll()
function. It makes the API more UNIX-like. How bad could the latency be?

tool/viz/lib/BUILD.mk

@@ -36,6 +36,7 @@ TOOL_VIZ_LIB_A_DIRECTDEPS =				\
 	LIBC_RUNTIME					\
 	LIBC_STDIO					\
 	LIBC_STR					\
+	LIBC_THREAD					\
 	LIBC_SYSV					\
 	LIBC_TESTLIB					\
 	LIBC_TINYMATH					\