Workaround for broken clang which has a broken -fno-zero-initialized-in-bss

Workaround for broken clang which has a broken -fno-zero-initialized-in-bss
implementation.

Set nkmempages to -1 by default instead of 0 so that the value ends up in
the data section. This way config(8) is able to alter the value as promised.

See also: https://github.com/llvm/llvm-project/issues/74632
OK miod@