parisc: Build kernel without -ffunction-sections

As suggested by Nick Piggin it seems we can drop the -ffunction-sections
compile flag, now that the kernel uses thin archives. Testing with 32-
and 64-bit kernel showed no difference in kernel size.

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>