enable gcc stack usage limit for kernel functions, set to 3.5 KiB for now
as that seems to be enough to accomodate the current biggest stack usages

there are about six functions which use over 3KiB local stack, and
about a dozen between 2-3 KiB, so pushing this further needs more work
if desired

compile tested on amd64, i386, sparc64, sparc, powerpc (evbppc - BookE),
m68k (mac68k)

(jdolecek)