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/sys_procfs_test.cpp b/tests/sys_procfs_test.cpp
index 5e0a0b0..4a64742 100644
--- a/tests/sys_procfs_test.cpp
+++ b/tests/sys_procfs_test.cpp
@@ -16,6 +16,7 @@
 
 #include <gtest/gtest.h>
 
+#include <ucontext.h> // for NGREG on musl
 #include <sys/procfs.h>
 
 TEST(sys_procfs, types) {