Globally enable linker dead-code elimination on Unix (#128232)

-ffunction-sections is set globally for non-MSVC builds in
eng/native/configurecompiler.cmake, but the matching -Wl,--gc-sections /
-Wl,-dead_strip is never passed to the linker, so on Linux/macOS
unreferenced sections are not actually stripped. Windows is unaffected
because /OPT:REF is set globally.

`eng/native/configurecompiler.cmake`:

- **Compile**: add `-fdata-sections` alongside the existing
`-ffunction-sections` (Unix/WASI) so unreferenced data, not just
functions, can be stripped.
- **Link, Apple**: `-Wl,-dead_strip` for Checked / Release /
RelWithDebInfo.
- **Link, other Unix **:
`-Wl,--gc-sections` for the same three configurations.
- **Debug**: untouched, matching Windows configuration

Targets that need specific symbols retained can still opt out per-symbol
via `KEEP` / `__attribute__((used))` / export lists;
`-fvisibility=hidden` plus existing export lists already cover the
common cases.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: elinor-fung <47805090+elinor-fung@users.noreply.github.com>
Co-authored-by: Elinor Fung <elfung@microsoft.com>
Co-authored-by: Adeel Mujahid <3840695+am11@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: 5 people

eng/native/configurecompiler.cmake

@@ -740,8 +740,15 @@ if (CLR_CMAKE_HOST_UNIX OR CLR_CMAKE_HOST_WASI)
   # We mark the function which needs exporting with DLLEXPORT
   add_compile_options(-fvisibility=hidden)
 
-  # Separate functions so linker can remove them.
+  # Separate functions and data into their own sections so the linker can remove
+  # unreferenced ones.
   add_compile_options(-ffunction-sections)
+  add_compile_options(-fdata-sections)
+  if(LD_OSX)
+    add_linker_flag(-Wl,-dead_strip CHECKED RELEASE RELWITHDEBINFO)
+  elseif(NOT LD_SOLARIS)
+    add_linker_flag(-Wl,--gc-sections CHECKED RELEASE RELWITHDEBINFO)
+  endif()
 
   # Specify the minimum supported version of macOS
   # Mac Catalyst needs a special CFLAG, exclusive with mmacosx-version-min