x86: Make x32 syscall support conditional on a kernel parameter

x86: Make x32 syscall support conditional on a kernel parameter

Enabling x32 in the standard amd64 kernel would increase its attack
surface while provide no benefit to the vast majority of its users.
No-one seems interested in regularly checking for vulnerabilities
specific to x32 (at least no-one with a white hat).

Still, adding another flavour just to turn on x32 seems wasteful.  And
the only differences on syscall entry are a few instructions that mask
out the x32 flag and compare the syscall number.

Use a static key to control whether x32 syscalls are really enabled, a
Kconfig parameter to set its default value and a kernel parameter
"syscall.x32" to change it at boot time.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic features/x86
Gbp-Pq: Name x86-make-x32-syscall-support-conditional.patch