Fix libbase compiling against musl

Musl doesn't have tgkill, define it in libbase.

Bug: 190084016
Test: m USE_HOST_MUSL=true
Change-Id: Ibc29c5850f70cb4840807c06d0bc96c1159df2c3
diff --git a/threads.cpp b/threads.cpp
index 48f6197..6d7d7c6 100644
--- a/threads.cpp
+++ b/threads.cpp
@@ -47,7 +47,7 @@
 }  // namespace base
 }  // namespace android
 
-#if defined(__GLIBC__)
+#if defined(__GLIBC__) || defined(ANDROID_HOST_MUSL)
 int tgkill(int tgid, int tid, int sig) {
   return syscall(__NR_tgkill, tgid, tid, sig);
 }