This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

This patch fixes:

randomness.h and getauxval.cpp were passing ssize_t as size_t
kernel_statx.h was assigning an uint64_t to uintptr_t
fopencookie.cpp was trying to create a FileIOResult using ssize_t but the constructor expected a size_t
thread.h was trying to call free_stack (which takes a size_t) with an unsigned long long. free_stack does the calculations using uintptr_t, so I changed the passing values to size_t

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.

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