um: Create asm/sections.h

arch/um/kernel/dyn.lds.S and arch/um/kernel/uml.lds.S define some
UML-specific symbols.  These symbols are used in the kernel part of UML
with extern declarations.

Move these declarations to a new header, asm/sections.h, like other
architectures do.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Richard Weinberger <richard@nod.at>