#4147 Fix stdbit.h issues and add PIC16 stdbit runtime support

[zeynelsagcan]

This patch fixes several issues in stdbit.h and makes the C23 bit utility helpers usable from the PIC16 library build.

Main fixes:

• repair malformed typed stdc_* macros and wrong helper references;
• fix incorrect inversions in stdc_first_leading_ / stdc_first_trailing_;
• add missing _BitInt entries in __typewidth() and fix the _BitInt(56) width typo;
• guard _BitInt-specific generic selections when the target does not support them;
• add __stdc_bit_floorull() to avoid the previous shift-by-width implementation;
• introduce __SDCC_ULLONG_WIDTH, using ULLONG_WIDTH where available and a C11-compatible fallback otherwise;
• add the existing __stdc_* helper sources to the PIC16 libc18f.a build.

Validation:

• all 8 __stdc_* helpers compile successfully for PIC16 with the normal --std-c11 library flags;
• the rebuilt libc18f.a contains all helper objects;
• an end-to-end PIC18F452 link using stdc_count_ones_ull() succeeds and produces a HEX file;
• stdbit_all.c parses successfully for PIC16 in C23 mode;
• __typewidth() was checked for missing and mismatched _BitInt(N) : N entries.

[Maarten Brock]

Apart from the copyright in __stdc_bit_floorull.c this looks good to me.
I doubt @spth Philipp sent you that file for you to attach it here.
Though I do understand the code was practically copied from stdbit.h.
Can the two of you please resolve this?

[zeynelsagcan]

Thanks, Maarten. You're right. Philipp did not send me this file. I created __stdc_bit_floorull.c using one of the existing _stdc* helper files as a template and accidentally left its copyright line in the new file. I'll correct that. I have attached the corrected file.

[Philipp Klaus Krause]

• I don't see the purpose of __SDCC_ULLONG_WIDTH - what's wrong with using ULLONG_WIDTH in the .c files? After all, those .c files are part of our own library and compiled as C11 or later. Also, I don't think we should #include <limits.h> in stdbit.h, since that would #define macros that the C standard does not list as defined for stdbit.h.

• You mentioned some inversion being incorrect. Could you add regression test for them in sdcc/support/regression/tests? Ideally also including some tests that use the types that were affected by the __typewidth issues, so the tests cover both.

• I've picked the _BitInt entry fixes in [r17062], and also added an entry for signed _BitInt(1).

[Maarten Brock]

• Note that ULLONG_WIDTH is only defined in limits.h for C23+, not for C11. But without including it this is no longer relevant. I have now added checks if ULLONG_WIDTH is defined and define it if it wasn't.
• You also might have picked the macros that are missing a closing parenthesis. I have done so now.
• I have removed signed _BitInt(1) again because it triggers warning 315. Either prevent the warning, make it pedantic or drop it.

All in [r17065].

I have not yet added __stdc_bit_floorull.c.

[Philipp Klaus Krause]

I also had noticed the warning, though unfortunately only after the commit. I'll make signed _BitInt(1) conditional on the standard version - I think we need it for correct results of these functions for signed _BitInt(1), which would be allowed in C2y.

[Philipp Klaus Krause]

Done in [r17083].

[Maarten Brock]

@spth & @janet-chien will you come to an agreement on what to do with __stdc_bit_floorull.c, resolve it and close this one?