[ARM] Control v6 'global' bit via Linux PTE entries

Unfortunately, we can't use the "user" bit in the page tables to
control whether a page table entry is "global" or "asid" specific,
since the vector page is mapped as "user" accessible but is not
process specific.

Therefore, give direct control of the ARMv6 "nG" (not global)
bit to the mm layers.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>