tests: userspace: test syscall buffer validation

Tests system call memory buffer read/write validation using the
k_pipe_get() and k_pipe_put() calls from a userspace thread.
Specifically, this tests _SYSCALL_MEMORY_READ/WRITE checks
by the system call handler by attempting to read/write to a
kernel object.

write_kobject_user_pipe() attempts to write over a kernel object
by using the kernel object's location as the buffer to place
the data read from the pipe.

read_kobject_user_pipe() attempts to read a kernel object by using
the kernel object's location as the location of data to be placed
into the pipe.

Tested on qemu_x86 and frdm_k64, passes on both.

Signed-off-by: Joshua Domagalski <jedomag@tycho.nsa.gov>