package/qt5base: install arch-specific configuration

For some architectures, it is necessary to link with additional
libraries, such as -latomic for sparc.

Add a bit of support to make that easy; actual use will be introduced in
the following patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Cc: Peter Seiderer <ps.report@gmx.net>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>