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/spawn_test.cpp b/tests/spawn_test.cpp
index d7ed970..3309466 100644
--- a/tests/spawn_test.cpp
+++ b/tests/spawn_test.cpp
@@ -31,7 +31,7 @@
 # if !defined(POSIX_SPAWN_SETSID)
 #  define POSIX_SPAWN_SETSID 0
 # endif
-#else
+#elif defined(__BIONIC__)
 #include <platform/bionic/reserved_signals.h>
 #endif
 
@@ -379,7 +379,7 @@
 }
 
 TEST(spawn, posix_spawn_POSIX_SPAWN_SETSIGMASK) {
-#if defined(__GLIBC__)
+#if defined(__GLIBC__) || defined(MUSL)
   GTEST_SKIP() << "glibc doesn't ignore the same signals.";
 #else
   // Block SIGBUS in the parent...
@@ -417,7 +417,7 @@
 }
 
 TEST(spawn, posix_spawn_POSIX_SPAWN_SETSIGDEF) {
-#if defined(__GLIBC__)
+#if defined(__GLIBC__) || defined(MUSL)
   GTEST_SKIP() << "glibc doesn't ignore the same signals.";
 #else
   // Ignore SIGALRM and SIGCONT in the parent...