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

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