sparc: Stash orig_i0 into %g6 instead of %g2

As per the comments added by this commit, %g2 turns out to not be a
usable place to save away orig_i0 for syscall restart handling.

In fact all of %g2, %g3, %g4, and %g5 are assumed to be saved across
a system call by various bits of code in glibc.

%g1 can't be used because that holds the syscall number, which would
need to be saved and restored for syscall restart handling too, and
that would only compound our problems :-)

This leaves us with %g6 and %g7 which are for "system use".  %g7 is
used as the "thread register" by glibc, but %g6 is used as a compiler
and assembler temporary scratch register.  And in no instance is %g6
used to hold a value across a system call.

Therefore %g6 is safe for storing away orig_i0, at least for now.

Signed-off-by: David S. Miller <davem@davemloft.net>