This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

reland of https://github.com/llvm/llvm-project/pull/98215

Additionally adds proxy headers for FILE and the fopencookie types

The arm32 build has been failing due to redefinitions of the off_t type.
This patch fixes this by moving off_t to a proper proxy header. To do
this, it also moves stdio macros to a proxy header to hopefully avoid
including this proxy header alongside this public stdio.h.

Reverts llvm/llvm-project#98215

Breaks linux bots

The arm32 build has been failing due to redefinitions of the off_t type.
This patch fixes this by moving off_t to a proper proxy header. To do
this, it also moves stdio macros to a proxy header to hopefully avoid
including this proxy header alongside this public stdio.h.

This patch includes changes related to the use of off_t in libc,
targeted at 32-bit systems: in several places, the offset is used either
as a long or an off_t (64-bit signed int), but in 32-bit systems a long
type is only 32 bits long.

Fix a warning in mmap where a long offset is expected, but we were
passing an off_t. A static_cast and a comment were added to explain
that we know we are ignoring the upper 32-bit of the off_t in 32-bit
systems.
The code in pread and pwrite was slightly improved to remove a
#ifdef LIBC_TARGET_ARCH_IS_RISCV32; we are using an if constexpr now.
The Linux file operations were changed to use off_t instead of a long
where applicable. No changes were made to the standard API, e.g.,
ftell returns the offset as an int so we added a static_cast and a
comment explaining that this will cause a loss of integer precision
in 32-bit systems.

Fixes #93711 .
This patch implements the ``fdopen`` function. Given that ``fdopen`` 
internally calls ``fcntl``, the implementation of ``fcntl`` has been
moved to the ``__support/OSUtil``, where it serves as an internal public
function.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This is done so that tests which only require platform file but not the
platform streams can be run as unit tests. The tests which use platform
streams can only be hermetic tests to avoid conflicts with the
system-libc streams.

Fixes:
https://github.com/llvm/llvm-project/issues/64663
https://github.com/llvm/llvm-project/issues/63558

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D158193