Add an option to statically link a custom libc++.

MSan and TSan both require instrumenting everything. Add some machinery so we
can do this on the bots.

Change-Id: I7d2106bc852ee976455d18787d3a20a35373a9e7
Reviewed-on: https://boringssl-review.googlesource.com/30644
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/.gitignore b/.gitignore
index bea7c55..db50b0b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,8 @@
 util/bot/cmake-win32.zip
 util/bot/golang
 util/bot/gyp
+util/bot/libcxx
+util/bot/libcxxabi
 util/bot/libFuzzer
 util/bot/llvm-build
 util/bot/nasm-win32.exe