[ARM] 3335/1: Old-abi Thumb sys_syscall broken

Patch from Paul Brook

The old-abi sys_syscall syscall is broken when called from Thumb mode. It
assumes the syscall number is an Arm syscall number (ie. starts from
__NR_OABI_SYSCALL_BASE).  In thumb mode syscall numbers start from zero.

The patch below fixes this by clearing the nigh bits of the syscall number
instead of inverting them. Technically this means we accept some invalid
syscall numbers, but I can't see how that could be a problem. The two sets of
numbers far apart that unimplemented syscalls should still be rejected.

Signed-off-by: Paul Brook <paul@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>