libthr: avoid gcc -Wcast-function-type

The actual types of pthread_func_t and interpos_func_t are not used.
Declare them as the generic void (*)(void) which suppresses warnings.

https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wcast-function-type

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45830