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/stdio_test.cpp b/tests/stdio_test.cpp
index 5736e17..9d840f5 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -2740,8 +2740,8 @@
 }
 
 TEST(STDIO_TEST, renameat2) {
-#if defined(__GLIBC__)
-  GTEST_SKIP() << "glibc doesn't have renameat2 until 2.28";
+#if defined(__GLIBC__) || defined(MUSL)
+  GTEST_SKIP() << "glibc doesn't have renameat2 until 2.28 and musl doesn't have renameat2";
 #else
   TemporaryDir td;
   android::base::unique_fd dirfd{open(td.path, O_PATH)};