Avoid specifying VM_PROT_EXECUTE in mappings from pipe_map and exec_map.

These submaps are used for mapping pipe buffers and execv() argument
strings respectively, so there's no need for such mappings to have
execute permissions.

Reported by: jhb
Reviewed by: alc, jhb, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17827