[libc] Move the definitions of the standard IO streams to the platform

This patch moves the definitions of the standard IO streams to the
platform file definition. This is necessary because previously we had a
level of indirection where the stream's `FILE *` was initialized based
on the pointer to the internal `__llvm_libc` version. This cannot be
resolved ahead of time by the linker because the address will not be
known until runtime. This caused the previous implementation to emit a
global constructor to initialize the pointer to the actual `FILE *`. By
moving these definitions so that we can bind their address to the
original file type we can avoid this global constructor.

This file keeps the entrypoints, but makes them empty files only
containing an external reference. This is so they still appear as
entrypoints and get emitted as declarations in the generated headers.

Reviewed By: lntue, sivachandra

Differential Revision: https://reviews.llvm.org/D152983

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