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

Reverts the revert #119295 of #118882 by expanding #118882 with
additional fixes which made CI unhappy.

Reverts llvm/llvm-project#118882

Several functions are now missing necessary types in fullbuild, e.g.
`off_t`, `ssize_t`. Reverting for now.

Fixes failures like this which I experienced:
```
FAILED: src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o
/nix/store/2r2xi5pbg29bsmqywsm5zgl8l7adky4i-gcc-wrapper-13.3.0/bin/g++ -DLIBC_NAMESPACE=__llvm_libc -D_DEBUG -I/build/libc-src-20.0.0-unstable-2024-12-05/libc -isystem /build/libc-src-20.0.0-unstable-2024-12-05/libc/build/include -g -std=gnu++17 -DLIBC_QSORT_IMPL=LIBC_QSORT_QUICK_SORT -DLIBC_ADD_NULL_CHECKS -DLIBC_COPT_PUBLIC_PACKAGING -MD -MT src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o -MF src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o.d -o src/unistd/linux/CMakeFiles/libc.src.unistd.linux.read.dir/read.cpp.o -c /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp
次のファイルから読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/OSUtil/linux/syscall.h:13,
         次から読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/OSUtil/syscall.h:15,
         次から読み込み:  /build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:11:
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:20:29: エラー: ‘ssize_t __llvm_libc::read(int, void*, size_t)’ が外部シンボル ‘read’ の別名となっています
   20 | LLVM_LIBC_FUNCTION(ssize_t, read, (int fd, void *buf, size_t count)) {
      |                             ^~~~
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/__support/common.h:46:34: 備考: in definition of macro ‘LLVM_LIBC_FUNCTION_IMPL’
   46 |   decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];                   \
      |                                  ^~~~
/build/libc-src-20.0.0-unstable-2024-12-05/libc/src/unistd/linux/read.cpp:20:1: 備考: in expansion of macro ‘LLVM_LIBC_FUNCTION’
   20 | LLVM_LIBC_FUNCTION(ssize_t, read, (int fd, void *buf, size_t count)) {
      | ^~~~~~~~~~~~~~~~~~
```

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

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 fixes the following compilation error: no known conversion from 'off_t *'
(aka 'long long *') to 'long' for 5th argument.

Since pointers are 32-bit long anyway, casting it to long shouldn't be a
problem. Tested on rv32.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157792