fix(libc): guard _jp2uc_l/_uc2jp_l calls in towctrans_l with _MB_CAPABLE

[esaurez]

Summary

newlib/libc/ctype/towctrans_l.c called _jp2uc_l and _uc2jp_l unconditionally, but those helpers (defined in jp2uc.c) are only compiled when _MB_CAPABLE is defined. In any newlib configuration without _MB_CAPABLE, linking anything that pulls in towctrans_l.o fails with undefined references to _jp2uc_l and _uc2jp_l. This blocks --whole-archive libc.a packaging for the nanvix targets.

This patch guards the Unicode/JIS machinery in towctrans_l.c with #ifdef _MB_CAPABLE and provides a clean no-MB fallback that matches the existing sibling functions.

Why this shape

Every other caller of _jp2uc_l/_uc2jp_l in newlib already guards the call behind #ifdef _MB_CAPABLE:

• iswalnum_l.c, iswalpha_l.c, iswlower_l.c, iswupper_l.c, iswprint_l.c, iswpunct_l.c, iswspace_l.c, iswgraph_l.c, iswblank_l.c, iswcntrl_l.c
• wcwidth.c, wcswidth.c

towctrans_l.c was the lone outlier. The fix restores symmetry.

For the no-MB fallback, the patched code delegates to towlower(c) / towupper(c). This matches what towlower_l.c and towupper_l.c already do in their own no-_MB_CAPABLE branch:

// towlower_l.c
#ifdef _MB_CAPABLE
  return towctrans_l (c, WCT_TOLOWER, locale);
#else
  return towlower (c);
#endif

So direct callers of towctrans / towctrans_l now see exactly the same result as towlower_l / towupper_l in the same configuration, instead of a Unicode-table-based answer that would diverge from every other no-MB wide-ctype function.

Wrapping the caseconv_table, bisearch, toulower, and touupper static helpers in the same #ifdef _MB_CAPABLE also avoids -Wunused-function on the otherwise-dead code in the no-MB build.

What changes

• _MB_CAPABLE defined (existing default): the preprocessor selects exactly the same code as before — towctrans_l is byte-for-byte the original implementation. The full caseconv_table and Unicode bisearch are still emitted.
• _MB_CAPABLE undefined (nanvix default): towctrans_l delegates to towlower / towupper for WCT_TOLOWER / WCT_TOUPPER, returns c for any other wctrans_t. No reference to _jp2uc_l / _uc2jp_l.

Verification

Cross-compiled towctrans_l.c with i686-nanvix-gcc from the nanvix toolchain image in both configurations:

Build	Result
Without _MB_CAPABLE	T towctrans_l; references towlower / towupper; no undefined _jp2uc_l / _uc2jp_l
With -D_MB_CAPABLE	T towctrans_l; U _jp2uc_l, U _uc2jp_l; d caseconv_table (original behavior)

Upstream note

The same bug exists in upstream sourceware newlib (mirror/newlib-cygwin master). This fix is being landed in nanvix/newlib first to unblock dependent work; a parallel proposal to upstream is planned as a follow-up.