[compiler-rt][CMake] Set project for the custom libc++

This is another follow up to r354212 which is broken on Darwin when
cross-compiling runtimes to Linux when it ignores the -fuse-ld=lld
linker flag and attempts to use the host linker when performing the
compiler identification. Upon investigation, I noticed that setting
the project with appropriate list of languages makes the error go
away and it shouldn't hurt either.

Differential Revision: https://reviews.llvm.org/D58372

llvm-svn: 354350

compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt

@@ -1,4 +1,5 @@
 cmake_minimum_required(VERSION 3.4.3)
+project(custom-libcxx C CXX)
 
 # Build static libcxxabi.
 set(LIBCXXABI_STANDALONE_BUILD 1)