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/SignalUtils.h b/tests/SignalUtils.h
index a2faf0a..f04ea59 100644
--- a/tests/SignalUtils.h
+++ b/tests/SignalUtils.h
@@ -19,7 +19,7 @@
 #include <signal.h>
 #include <string.h>
 
-#if defined(__GLIBC__)
+#if !defined(__BIONIC__)
 #define posix_spawnattr_getsigdefault64 posix_spawnattr_getsigdefault
 #define posix_spawnattr_getsigmask64 posix_spawnattr_getsigmask
 #define posix_spawnattr_setsigdefault64 posix_spawnattr_setsigdefault