Keep local symbols of rump libraries

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

I'm using ATF tests running rump kernels (rump_server)
for development and debugging. When a rump kernel gets
panic, it dumps a core file from which we can obtain
a backtrace by using gdb.

Unfortunately local symbols (i.e., static functions)
in a backtrace are unresolved because they are stripped
by the linker (and objdump). That makes debugging a bit
difficult.

The patch introduces a compile option for rump kernels
called RUMP_KEEPSYMBOLS to keep such symbols:
  http://www.netbsd.org/~ozaki-r/rumplibs-keep-symbols.diff

The option is disabled by default to not increase
the size of all rump libraries.

I'm not so familiar with the NetBSD build system and
Makefiles, so the patch may be wrong or clumsy.

Any comments?

Thanks,
  ozaki-r