[libc] Define SSIZE_MAX in limits-macros.h (#201087)
Defined SSIZE_MAX in limits-macros.h to support POSIX compliance.
Applications compiling against LLVM libc require SSIZE_MAX to be defined
in limits.h.
SSIZE_MAX is defined strictly in terms of __PTRDIFF_MAX__, matching
ssize_t defined as __PTRDIFF_TYPE__, ensuring they are always
structurally aligned and compile-time safe.
Added Doxygen comments for SSIZE_MAX.
Added SSIZE_MAX to limits.yaml and added a unit test in limits_test.cpp
to verify the definition.
Assisted-by: Automated tooling, human reviewed.
