Change qt_ucstrncmp_mips_dsp_asm()'s length parameter to size_t

Please change the length parameter of the qt_ucstrncmp_mips_dsp_asm() function from uint (always 32 bits) to size_t (32 or 64 bits, depending on the processor's word size).

The introduction of QStringView (https://codereview.qt-project.org/143763) necessitates extending internal string handling functions to be able to take more than 2 Gi of characters, so I'll likely need the same change in the from/toLatin1 functions.

I will work around the issue by looping over the functions in blocks of UINT_MAX, but it would be nice if you could change the assembler functions eventually.