Create proper debug configuration

[icegood]

Description (original from @icegood)

With this PR it is easier to create more rigorous debug builds for further debugging in gdb. (for at least spk/ffmpeg)
Not applicable out of the box since other dependant repos need to be fit as well.

To check against: https://github.com/icegood/spksrc/tree/fix/libpng_debug_builds at it fixes png bug

Applicable for issue #6176

Additions (from @th0ma7)

1. Dissociate CMake toolchain file generation from default spksrc.cross-cmake.mk (similar to meson)
2. Enforce using an "empty" environment for both CMake and meson (fix issue against 'build' vs 'host' builds)
3. Fix meson crossfile generation - needs [built-in options] and fortran_* instead of fc_*
4. Fix CMake toolchain file generation with regards to ldshared definition
5. Add CMAKE_*_FOR_BUILD to CMake toolchain definition
6. Enable enforcing passing proper debug flags throughout CMake and Meson builds
7. Add passing debug flags to ENV with autoconf/autotools
8. Fixed a few cross/* Debug compile errors
9. Fixed situation where spksrc.copy.mk would not exit on error using tar from pipe
10. Updated cross/* when extra code was needed to allow debug build to work

Relates to #6719

Checklist

• Build rule all-supported completed successfully
• New installation of package completed successfully
• Package upgrade completed successfully (Manually install the package again)
• Package functionality was tested
• Any needed documentation is updated/created

Type of change

• Bug fix
• New Package
• Package update
• Includes small framework changes
• This change requires a documentation update (e.g. Wiki)

Tests

Tested in debug mode against:

• synocli-videodriver
• ffmpeg
• python312
• tvheadend

[th0ma7]

First off, thnx a lot for your contribution. You actually went quite a bit into the weeds in here, well done. You'll find a a few questions, suggestions & changes proposed part of my review.

[th0ma7]

@hgy59 Due to a crash with tvheadend I needed to generate a dull Debug build of its entire stack... i recalled the nice work from @icegood that I reused and enhanced and with it I can now generate a fully consistent debug build with proper debug_info sympbols. This is turn allow proper gdb usage to track exactly where the issue is.

If you don't mind having a look at this - and testing a few builds that you think may be useful to validate - would be real nice. Thnx.

[th0ma7]

@hgy59 i've hit a few corner cases, needs a bit more love before its fully baked.

[th0ma7]

@hgy59 I believe it is now ready for review and testing. I suggest you add GCC_DEBUG_INFO := 1 to spk you whishes to test in full. It should not impact normal (e.g. non-debug) builds although changes are a bit more agressive than anticipated.

EDIT: This PR is a hard dependency for #6719

[th0ma7]

Builds looks good, from my perspective this is now ready for merge.