UPSTREAM: ARM: 7475/1: sys_trace: allow all syscall arguments to be updated via ptrace

(cherry pick from commit c7aa00db)

Prior to syscall invocation, __sys_trace only reloads r0-r3 from the
kernel stack, preventing the debugger from updating arguments 5-7 when
signalled via ptrace.

This patch updates the code to reload r0-r6, updating arguments 5 and 6
on the stack (argument 7 is only used by OABI indirect syscalls and
can remain in a register).

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Bug: 9674955
Bug: 25773781
Change-Id: Ic7714ab986d3745b10df207f757ea4af68b79552