make Program::Execute use posix_spawn on systems that support it,

make Program::Execute use posix_spawn on systems that support it,
as it is more efficient than fork/exec.

Thanks to Eric for adding the autoconf check.  It would be nice if
a cmake guru could add a cmake check for posix_spawn as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101693 91177308-0d34-0410-b5e6-96231b3b80d8