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

riscv32 currently doesn't have SYS_wait4, so wait4 is implemented via fallback
to SYS_waitid. In #125572, I missed that we had one use of the removed
__W_CONTINUED value. Hard code it here.

Fixes: #125572

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

sys/signal.h currently does not exist in LLVM-libc. Both glibc and
musl provides this header with simply:

> #include <signal.h>
(and a warning in musl libc)

If you try to build LLVM-libc in a sysroot with only LLVM-libc headers
and linux-headers installed then this will fail the build due to
missing headers. However, if --sysroot is not passed, then the
LLVM-libc build will pick up the header from the system which in turn
includes LLVM-libc's signal.h.

LLVM-libc includes <signal.h> in all other cases, so this simply
matches that. Providing <sys/signal.h> could also be done (instead)
for glibc compatibility.

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