kbuild: add '-fno-stack-check' to kernel build options

kbuild: add '-fno-stack-check' to kernel build options

It appears that hardened gentoo enables "-fstack-check" by default for
gcc.

That doesn't work _at_all_ for the kernel, because the kernel stack
doesn't act like a user stack at all: it's much smaller, and it doesn't
auto-expand on use.  So the extra "probe one page below the stack" code
generated by -fstack-check just breaks the kernel in horrible ways,
causing infinite double faults etc.

[ I have to say, that the particular code gcc generates looks very
  stupid even for user space where it works, but that's a separate
  issue.  ]

Reported-and-tested-by: Alexander Tsoy <[email protected]>
Reported-and-tested-by: Toralf Förster <[email protected]>
Cc: [email protected]
Cc: Dave Hansen <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>