Add build hook to copy .s files

Previously, the project relied on undefined behavior of older versions of the Arduino build system, where correct file
extension case was not enforced when copying files from the sketch folder to the build folder, in order to .

This stopped working at Arduino IDE 1.8.14, meaning that users were forced to use a significantly outdated version of
the IDE; a requirement which was undocumented.

This is fixed by adding an additional build hook to platform.local.txt that copies the `.s` files from the sketch folder
to the build folder.

Due to limitations of the Arduino platform framework, it was necessary to jump through some hoops to get working
patterns for these commands:

- Windows command pattern must use `cmd /C` because `copy` is not directly usable via the os/exec Go package that
  executes the generated commands.
- Linux/macOS command patterns must use `cd` because otherwise the `*.s` glob was broken by being automatically quoted
  as part of the sketch path.
- Linux/macOS command patters must use `sh -c` because `cd` is not directly usable via the os/exec Go package that
  executes the generated commands.

platform.local.txt

@@ -16,6 +16,11 @@ compiler.c.elf.extra_flags="-L{build.path}/sketch/" -T ulp_main.ld "{build.path}
 ## add ulptool.h to g++
 compiler.cpp.extra_flags="-I{tools.ulptool.path}/include/ulptool"
 
+## copy '.s' (ulp) files from sketch to build folder
+recipe.hooks.core.postbuild.00.pattern.windows=cmd /C copy {build.source.path}\*.s "{build.path}\sketch"
+recipe.hooks.core.postbuild.00.pattern.linux=sh -c 'cd "{build.source.path}" && cp *.s "{build.path}/sketch"'
+recipe.hooks.core.postbuild.00.pattern.macosx=sh -c 'cd "{build.source.path}" && cp *.s "{build.path}/sketch"'
+
 ## compile '.s' (ulp) files
 recipe.hooks.core.postbuild.01.pattern={compiler.s.cmd} {compiler.cpreprocessor.flags} -b {build.path} -p {runtime.platform.path} -u {compiler.ulp.path} -x {compiler.path} -t {tools.ulptool.path} --DF_CPU={build.f_cpu} --DARDUINO={runtime.ide.version} --DARDUINO_={build.board} --DARDUINO_ARCH_={build.arch} --DARDUINO_BOARD="{build.board}" --DARDUINO_VARIANT="{build.variant}"