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.

When building with compiler-rt enabled, warnings such as the following
are
observed:


llvm-project/llvm/build/projects/compiler-rt/../libc/include/llvm-libc-macros/linux/sys-stat-macros.h:46:9:
    warning: 'S_IXOTH' macro redefined [-Wmacro-redefined]
    #define S_IXOTH 00001
            ^

llvm-project/llvm/build/projects/compiler-rt/../libc/include/llvm-libc-macros/linux/fcntl-macros.h:61:9:
    note: previous definition is here
    #define S_IXOTH 01
            ^
It looks like we have these multiply defined. Deduplicate these flags;
users
should expect to find them in sys/stat.h. S_FIFO was wrong anyways
(should
have been S_IFIFO).

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

In patch D157792, the calls to SYS_llseek/SYS_llseek for 32-bit systems
were fixed in lseek.cpp but there was another implementation in file.cpp
that was missed.

To reduce the code duplication, this patch unifies both call sites to
use a new lseekimpl function.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159208

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

This patch is large, but is almost entirely just adding casts to calls
to syscall_impl. Much of the work was done programatically, with human
checking when the syntax or types got confusing.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D156950

This patch adds a bunch of ifdefs to handle the 32 bit versions of
some syscalls, which often only append a 64 to the name of the syscall
(with exception of SYS_lseek -> SYS_llseek and SYS_futex ->
SYS_futex_time64)

This patch also tries to handle cases where wait4 is not available
(as in riscv32): to implement wait, wait4 and waitpid when wait4 is
not available, we check for alternative wait calls and ultimately rely
on waitid to implement them all.

In riscv32, only waitid is available, so we need it to support this
platform.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148371

Before this change, a separate static method named cleanup was used to
cleanup the file. Instead, now the close method cleans up the full file
object using the platform's close function.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D153377

The libc flush operation is not supposed to trigger a platform level
flush operation. See "Notes" on this Linux man page:
    https://man7.org/linux/man-pages/man3/fflush.3.html

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D153182

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