Declare pthread_atfork as weak to avoid a fatal error with LLVM 13

Declare pthread_atfork as weak to avoid a fatal error with LLVM 13

Fixes behavior with current clang, which marks the symbol as GLOBAL
instead of WEAK.  LLVM change: https://reviews.llvm.org/D90108
base-gcc is unaffected.  Keep asm(".weak") for gcc3 until a cleanup can
be tested there.

Initial diff from mortimer@, input and ok kettenis@, ok guenther@