[sanitizer_common] Always use posix_spawn on Darwin

On Darwin we have two external symbolizers: atos and llvm-symbolizer.
atos was changed to use posix_spawn (instead of fork+execv) in a
previous commit [1]. Let's use posix_spawn for llvm-symbolizer as well.

Our hope is that eventually we can transition to posix_spawn on other
platforms too.

[1] 399408a9

llvm-svn: 369021