Add interfaces to dynamically change symbol tables used by posix_spawn, execv,...

Add interfaces to dynamically change symbol tables used by posix_spawn, execv, and execl.  This is needed for testing.

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5506 42af7a65-404d-4744-a932-0658087f49c3