Fix CPU_COUNT check for FreeBSD 13.1+

[Thefrank]

HAVE_GNU_CPU_COUNT has always incorrectly failed for FreeBSD.

This has not been an issue until FreeBSD 13.1 with its addition of sched_getaffinity and sched_setaffinity.

[Thefrank]

more info: With the two sched checks passing but CPU_COUNT failing it will try and make its own CPU_COUNT...and fail because FreeBSD already has a macro it uses for that.

Thinking about it more, the other option would be to opt FreeBSD out of HAVE_SCHED_GETAFFINITY and HAVE_GNU_CPU_COUNT

[Thefrank]

Switched to check_symbol_exists as per @am11 suggestion.
I left it in the section with the other "GNU" items but for readability it might be better with the other check_symbol_exists items

For those wondering:
FreeBSD 13.1:
CPU_COUNT sys/cpuset.h:74:#define CPU_COUNT(p) ((int)__BIT_COUNT(CPU_SETSIZE, p))
Ubuntu 20.04.02 + kernel 5.18.4
CPU_COUNT sched.h:88: # define CPU_COUNT(cpusetp) __CPU_COUNT_S (sizeof (cpu_set_t), cpusetp)

[am11]

LGTM

[akoeplinger]

Thank you!