Enable coverage for libc.

Bug: http://b/157081822

- Disable coverage for libtest_check_rtld_next_from_library to make
dlfcn.rtld_next_from_library to pass.
- The coverage runtime sets an environment variable to prevent
clobbering of the coverage data on disk.  Include that variable as well
when checking `printenv` output.  This is done at runtime by checking
for LLVM_PROFILE_FILE environment variable.
- dl.preinit_system_calls still fails after this change.

Test: bionic tests with libc coverage turned on.

Change-Id: I3ea2b0800886d8c0984969a4ee8bfb0da03c33b0
diff --git a/tests/libs/Android.bp b/tests/libs/Android.bp
index c427282..fcf4497 100644
--- a/tests/libs/Android.bp
+++ b/tests/libs/Android.bp
@@ -1268,6 +1268,7 @@
     name: "libtest_check_rtld_next_from_library",
     defaults: ["bionic_testlib_defaults"],
     srcs: ["check_rtld_next_from_library.cpp"],
+    native_coverage: false,
 }
 
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