libffi: hide platform-ignorant FFI ABI enum values

[drcicero]

This change ignores processing the whole ffitarget.h file, which If i understood correctly contains platform-specific values, but only for one platform, and therefore is on most platforms wrong. I believe the platform-specific values can already be accessed native methods handwritten in the preset. I added the otherwise missing FFI_TRAMPOLINE_SIZE() method.

I think only two other variables remain in the ffitarget.h file, which are FFI_CLOSURES = 1; FFI_GO_CLOSURES = 1;. I'm not super sure, but i believe those are only there to enable/disable definitions in the other header, it shouldn't hurt not to have them.

I tried mvn clean test --projects .,libffi and it succeeds. I have not figured out if there are more tests to run?

See #1315

[drcicero]

Oh, the example https://github.com/bytedeco/javacpp/wiki/Mapping-Recipes#skipping-lines-from-header-files sounded like one can only skip lines of the form

// START COMPLEX DECLARATIONS
// ...
// END COMPLEX DECLARATIONS

but by studying the other InfoMaps, I found it is actually possible to skip individual definitions as well.

I suppose a simpler change would be to add the following:

.put(new Info("FFI_FIRST_ABI","FFI_SYSV","FFI_THISCALL","FFI_FASTCALL","FFI_STDCALL","FFI_PASCAL","FFI_REGISTER","FFI_MS_CDECL","FFI_LAST_ABI","FFI_DEFAULT_ABI").skip())

[drcicero]

Now, the change is only the small addition of a few skips (.put(new Info("FFI_FIRST_ABI", "FFI_SYSV", "FFI_THISCALL", "FFI_FASTCALL", "FFI_STDCALL", "FFI_PASCAL", "FFI_REGISTER", "FFI_MS_CDECL", "FFI_LAST_ABI", "FFI_DEFAULT_ABI").skip())), because those ones already are defined in the preset. And as this leads to having the correct FFI_DEFAULT_ABI definition autogenerated, I removed the manual definition of FFI_DEFAULT_ABI from the preset.

[saudet]

For consistency though, we might as well add a skip for FFI_DEFAULT_ABI too and leave it with the rest in the manually declared ones.