BUILD: defaults: use __WORDSIZE not LONGBITS for MAX_THREADS_PER_GROUP

BUILD: defaults: use __WORDSIZE not LONGBITS for MAX_THREADS_PER_GROUP

LONGBITS was defined long ago with old compilers that didn't provide the
word size. It's still present as being referenced in various places in the
code, but we must not use it to define other macros that may be evaluated
at pre-processing time since it contains sizeof() and casts that are not
compatible with preprocessor conditions. Let's switch MAX_THREADS_PER_GROUP
to __WORDSIZE so that we can condition blocks of code on it if needed.

LONGBITS should really be removed by now, given that we don't support
compilers not providing __WORDSIZE anymore (gcc < 4.2).