pax: x86_64: fix copy_user_generic

Pax patch removed r10 from clobbers list as all assembly code was
updated to not use it. However, in the error path the assembly code
ends up calling copy_user_handle_tail() C-function that can clobber
r10. This broke things for certain gcc optimizations (at least for gcc
4.8.x), as copy_user_generic() is inline function, and the inline
assembly incorrectly guaranteed not to clobber r10.

Add r10 back to clobbers list like vanilla kernel has it. It is
needed always due to the possibly C-function call in error path.

Signed-off-by: Timo Teräs <timo.teras@iki.f>
Acked-by: Natanael Copa <ncopa@alpinelinux.org>