unix: add SSIZE_MAX constant for unix targets

[telcharr]

Closes #1504

Description

Adds the POSIX SSIZE_MAX constant at the unix level. Defined as ssize_t::MAX, since ssize_t is isize on all unix targets making it correct on 32-bit, 64-bit, and x32 without any cfg. Binding the named constant is still worth it for discoverability and parity with the C API.

This is a fixed POSIX type limit, not a *MAX enumeration sentinel of the kind #3131 targets, so it's not a placeholder constant.

Sources

• POSIX / Open Group <limits.h>:
  https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html

  {SSIZE_MAX} Maximum value for an object of type ssize_t.

• glibc posix/bits/poisx1_lim.h:
  https://github.com/bminor/glibc/blob/04e750e75b73957cf1c791535a3f4319534a52fc/posix/bits/posix1_lim.h#L164-L172

  #ifndef	SSIZE_MAX
  /* ssize_t is not formally required to be the signed type
     corresponding to size_t, but it is for all configurations supported
     by glibc.  */
  # if __WORDSIZE == 64 || __WORDSIZE32_SIZE_ULONG
  #  define SSIZE_MAX	LONG_MAX
  # else
  #  define SSIZE_MAX	INT_MAX
  # endif
  #endif

• musl include/limits.h:53:
  https://git.musl-libc.org/cgit/musl/tree/include/limits.h#n53

  #define SSIZE_MAX LONG_MAX

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated