Make gyp to dump debug symbols for separate_debug_info builds

With this patch we get usable debug files.
This also makes qmake strip the core lirary from unneeded sysmbols
which is especially beneficial on embedded.

Change-Id: Ic1cadd87dbedb9817512a8b4f251cc3e43f028d1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>