Build [Configure]: Prevent inling of find_stack_direction()

Clang 11 has either learned to inline find_stack_direction(), or it may
have inlined find_stack_direction() before but changed allocation order
for our "objects with automatic storage duration" that we use to check
the stack direction.

Calling find_stack_direction() through a volatile function pointer
should reliably prevent it from getting inlined and thus flush out the
real stack direction.

Thanks to Cezary Cerekwicki <ccerekwicki@opera.com> for the report.