mmap: Add an exception to the stack gap for Hotspot JVM compatibility

mmap: Add an exception to the stack gap for Hotspot JVM compatibility

The Hotspot JVM can easily exhaust the default stack, and has a
SIGSEGV handler to cope with this by switching to a new stack segment.

However, on i386 it creates a single writable and executable page just
under the stack limit as a workaround for a bug in Exec Shield.  That
together with the enlarged stack gap causes the SIGSEGV handler to be
triggered when the stack pointer is further away from the stack limit,
and it doesn't recognise this as being a stack overflow.

This specifically affects programs that use JNI.  Hotspot doesn't
normally run Java code on the initial thread.

Reduce the effective stack guard gap on x86 if the previous vma is
a single page allocated as MAP_FIXED.

References: https://bugs.debian.org/865303
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/x86
Gbp-Pq: Name mmap-add-an-exception-to-the-stack-gap-for-hotspot-jvm.patch