waf: allow cross-execute program to have arguments with spaces

waf: allow cross-execute program to have arguments with spaces

When cross-compiling samba using the cross-execute method,
allow the cross-excute command to have arguments that contain
spaces by quoting them.

For example:

./configure --cross-compile '--cross-execute=ce-program "par am"'

In this case, for each cross test whose binary is /path/testprog,
waf shall run the equivalent of running from a shell:
ce-program "par am" /path/testprog

This is useful for passing an arbitrary argument list to a subprocess
of the cross-execute program, such as in buildtools/examples/run_on_target.py

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>