[Fuzzer] Avoid the unnecessary rebuild of the custom libc++

This changes the add_custom_libcxx macro to resemble the
llvm_ExternalProject_Add. The primary motivation is to avoid
unnecessary libFuzzer rebuilds that are being done on every
Ninja/Make invocation. The libc++ should be only rebuilt whenever
the libc++ source itself changes.

Differential Revision: https://reviews.llvm.org/D43213

llvm-svn: 326921