criu/arch/x86/Makefile: move -Wa out of asflags

In a manner similar to one of commit 9303ed3 ("Makefiles: move
-Wa,--noexecstack out of CFLAGS"), let's move this option
from assembler to linker flags (see the abovementioned commit
for reasoning).

This fixes generating dependencies for criu/arch/x86/call32.S when using clang:

  DEP      criu/arch/x86/call32.d
clang-3.8: error: argument unused during compilation: '-Wa,--noexecstack'
  DEP      criu/arch/x86/call32.d
clang-3.8: error: argument unused during compilation: '-Wa,--noexecstack'

travis-ci: success for criu/arch/x86/Makefile: move -Wa out of asflags (rev2)
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>