compiler-rt/test: Clean up Android specific workarounds in lit.common.cfg.

[pcc]

Summary

-pie -Wl,--enable-new-dtags are no longer needed because
the driver passes them by default as of r316606.

Prepend -fuse-ld=gold instead of appending it so that the linker can
be overridden using COMPILER_RT_TEST_COMPILER_CFLAGS.

[eugenis]

LGTM

compiler-rt/test/lit.common.cfg

else:
   lit_config.fatal("Unsupported compiler id: %r" % compiler_id)
 # Add compiler ID to the list of available features.
 config.available_features.add(compiler_id)
 
 # If needed, add cflag for shadow scale.
 if config.asan_shadow_scale != '':
   config.target_cflags += " -mllvm -asan-mapping-scale=" + config.asan_shadow_scale
 
-# BFD linker in 64-bit android toolchains fails to find libm.so, which is a
-# transitive shared library dependency (via asan runtime).
+# BFD linker in 64-bit android toolchains fails to find libc++_shared.so, which
+# is a transitive shared library dependency (via asan runtime).
 if config.android:
-  config.target_cflags += " -pie -fuse-ld=gold -Wl,--enable-new-dtags"
+  # Prepend the flag so that it can be overridden.
+  config.target_cflags = "-fuse-ld=gold " + config.target_cflags
   config.cxx_mode_flags.append('-stdlib=libstdc++')
 
 # Clear some environment variables that might affect Clang.
 possibly_dangerous_env_vars = ['ASAN_OPTIONS', 'DFSAN_OPTIONS', 'LSAN_OPTIONS',
                                'MSAN_OPTIONS', 'UBSAN_OPTIONS',
                                'COMPILER_PATH', 'RC_DEBUG_OPTIONS',
                                'CINDEXTEST_PREAMBLE_FILE', 'LIBRARY_PATH',
                                'CPATH', 'C_INCLUDE_PATH', 'CPLUS_INCLUDE_PATH',