cmake: Explicitly set the entry point when testing toolchain flags

We are observing warnings when test-compiling toolchain flags that
look like:

cannot find entry symbol _start; defaulting to 00000000000010d4

To fix this warning we explicitly set an entry point to an arbitrary
address. The files are only compiled, not run, so the entry point does
not need to be correct.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>