[libc-commits] [libc] [libc][sys/wait][linux] add missing and clean up existing macros (PR #125572)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

ack, looking:
```
In file included from /home/libcrv32buildbot/bbroot/libc-riscv32-qemu-yocto-fullbuild-dbg/llvm-project/libc/src/sys/wait/linux/waitpid.cpp:13:
/home/libcrv32buildbot/bbroot/libc-riscv32-qemu-yocto-fullbuild-dbg/llvm-project/libc/src/sys/wait/wait4Impl.h:68:22: error: use of undeclared identifier '__W_CONTINUED'
   68 |       *wait_status = __W_CONTINUED;
      |                      ^
```

https://github.com/llvm/llvm-project/pull/125572