Add start of filesystem benchmarks

llvm-svn: 285524
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 3aa5478e212ced5f0f6ee61359d7ebbe167bbb10
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 1c3be87..b00bbdd 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -256,14 +256,21 @@
   Build the libc++ benchmark tests and the Google Benchmark library needed
   to support them.
 
-.. option:: LIBCXX_BUILD_BENCHMARK_NATIVE_STDLIB:BOOL
+.. option:: LIBCXX_BENCHMARK_NATIVE_STDLIB:STRING
 
-  **Default**:: ``OFF``
+  **Default**:: ``""``
+
+  **Values**:: ``libc++``, ``libstdc++``
 
   Build the libc++ benchmark tests and Google Benchmark library against the
-  native standard library on the platform. On linux this can be used to compare
-  libc++ to libstdc++ by building the benchmark tests against both standard
-  libraries.
+  specified standard library on the platform. On linux this can be used to
+  compare libc++ to libstdc++ by building the benchmark tests against both
+  standard libraries.
+
+.. option:: LIBCXX_BENCHMARK_NATIVE_GCC_TOOLCHAIN:STRING
+
+  Use the specified GCC toolchain and standard library when building the native
+  stdlib benchmark tests.
 
 
 libc++ ABI Feature Options