Enable posix strerror_r test in glibc

Work around the incompatibility between libc++ and !_GNU_SOURCE
by defining a wrapper function around the posix strerror_r in
a file that does not include any libc++ or gtest headers.

Test: bionic-unit-tests-glibc
Change-Id: I6b097079777b5dcd802e295cc566964fe1c01382
diff --git a/tests/Android.bp b/tests/Android.bp
index 476b8f5..0f9af41 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -406,6 +406,7 @@
         "string_nofortify_test.cpp",
         "string_test.cpp",
         "string_posix_strerror_r_test.cpp",
+        "string_posix_strerror_r_wrapper.cpp",
         "strings_nofortify_test.cpp",
         "strings_test.cpp",
         "struct_layout_test.cpp",