Build bionic unit tests for musl

Modify bionic unit tests that are built for glibc so that they also
build against musl.  They don't all pass though:

With glibc:
 2 SLOW TESTS
 4 TIMEOUT TESTS
313 FAILED TESTS
  YOU HAVE 2 DISABLED TESTS

With musl:
11 SLOW TESTS
11 TIMEOUT TESTS
363 FAILED TESTS
  YOU HAVE 2 DISABLED TESTS

Bug: 190084016
Test: m bionic-unit-tests-glibc with musl
Test: atest bionic-unit-tests-static
Test: atest --host bionic-unit-tests-glibc with glibc
Change-Id: I79b6eab04fed3cc4392450df5eef2579412edfe1
diff --git a/tests/leak_test.cpp b/tests/leak_test.cpp
index 4ebf41f..80618e5 100644
--- a/tests/leak_test.cpp
+++ b/tests/leak_test.cpp
@@ -21,6 +21,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <sys/syscall.h>
 #include <sys/user.h>
 #include <unistd.h>
 
@@ -44,7 +45,7 @@
     alive = false;
     for (size_t i = 0; i < tid_count; ++i) {
       if (tids[i] != 0) {
-        if (tgkill(getpid(), tids[i], 0) == 0) {
+        if (syscall(__NR_tgkill, getpid(), tids[i], 0) == 0) {
           alive = true;
         } else {
           EXPECT_EQ(errno, ESRCH);