fix(cmake): replace obsolete -s with -Wl,-x on Apple linker

[matejk]

Problem

Every Release link on macOS prints:

ld: warning: -s is obsolete

Apple's linker (ld64 / ld-prime, Xcode 15+) deprecated the -s flag introduced for size reduction in #5297. The link still strips, but the warning is emitted on every shared library and executable. LLVM's lld accepts -s silently — the warning is specific to Apple's ld.

Change

Gate the strip flag by platform in cmake/DefinePlatformSpecific.cmake:

• Apple: -Wl,-x — Apple's documented replacement. Strips local symbols while keeping dynamic exports needed for .dylibs.
• Other Unix (GCC/Clang on Linux/BSD): -s retained, unchanged.
• MSVC: intentionally no change. PE binaries do not carry a strippable symbol table — debug info lives in separate .pdb files. There is no -s equivalent on Windows.

Verification

Local rebuild on macOS (Xcode 16, Apple clang) of the Foundation/Util chain:

• ld: warning: -s is obsolete — gone.
• nm libPocoUtil.dylib shows 640 global text symbols, 0 local (lowercase) symbols → strip is effective.
• Util test suite: OK (244 tests).

Notes

• Out of scope: the /OPT:REF and /OPT:ICF flags on MSVC are dead-code/folding parallels of -Wl,-dead_strip / --gc-sections / --icf=safe, not of -s. They are MSVC linker defaults for Release already; making them explicit can be a separate follow-up if desired.
• Bit-for-bit equivalence to GNU -s would also pass -Wl,-S (strip debug info) on Apple, but clang on macOS emits DWARF into object files and only collects it into a .dSYM on demand, so the linked output rarely carries debug info; -Wl,-x alone matches the size-reduction goal.