[libc] Add custom operator new to handle allocation failures gracefully.

This patch adds the implementation of the custom operator new functions.
The implementation of the internal strdup has been updated to use
operator new for allocation.

We will make it a policy and document that all allocations have to go
through the libc's own operator new. A future change will also add
operator delete replacements and make it a policy that deallocations in
libc internal code have to go through those replacements.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D139584