arrow

GH-23221: [C++] Add support for building with Emscripten (#37821)

Split from #37696 

This is just the cmake changes to enable building on emscripten.

Changes are:
1) Support for target system "emscripten"
2) Cmake preset for building libarrow ` ninja-release-python-emscripten` (same as `ninja-release-python`, but with emscripten support)
3) Override file for cmake on emscripten, to set various build parameters that need setting to make it build there.
4) Changes in pyarrow cmake so it works if you are building libarrow as shared library, and also an option to enable the cmake file there to just dump the current arrow configuration, which is useful for cross-compile builds.

* Closes: #23221

Lead-authored-by: Joe Marshall <joe.marshall@nottingham.ac.uk>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>