um: increase stack growth cushion in pagefault

analog of [PATCH] i386: let usermode execute the "enter" instruction from
circa 2006.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

diff --cc arch/x86/um/asm/processor.h
Simple merge

diff --cc arch/x86/um/asm/processor_32.h
Simple merge

diff --cc arch/x86/um/asm/processor_64.h
Simple merge