libc: treat execvpe as a week symbol

Some program intercepts the execvpe call in runtime. At the same time, the
implementation of posix_spwan use execvpe. If execvpe is intercepted and
then calls posix_spawn when intercepted. It wil create a infinite loop
because the intercepted execvpe will spawn a posix_spwan call and will be
intercepted again.

See https://github.com/rizsotto/Bear/issues/557 for reference.

Reviewed by: brooks, kib, emaste
Sponsored by: FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49733