configure: add option to disable -fstack-protector flags

The -fstack-protector flag family is useful for ensuring safety and for
debugging, but has a performance impact. Here are some boot time comparisons of
the various versions of -fstack-protector using qemu-system-arm on an x86_64
host:

    # -fstack-protector-all
    Startup finished in 1.810s (kernel) + 12.331s (initrd) + 49.016s (userspace) = 1min 3.159s
    Startup finished in 1.801s (kernel) + 12.287s (initrd) + 47.925s (userspace) = 1min 2.013s
    Startup finished in 1.812s (kernel) + 12.302s (initrd) + 47.995s (userspace) = 1min 2.111s

    # -fstack-protector-strong
    Startup finished in 1.744s (kernel) + 11.223s (initrd) + 44.688s (userspace) = 57.657s
    Startup finished in 1.721s (kernel) + 11.222s (initrd) + 44.194s (userspace) = 57.138s
    Startup finished in 1.693s (kernel) + 11.250s (initrd) + 44.426s (userspace) = 57.370s

    # -fstack-protector
    Startup finished in 1.705s (kernel) + 11.409s (initrd) + 43.563s (userspace) = 56.677s
    Startup finished in 1.877s (kernel) + 11.137s (initrd) + 43.719s (userspace) = 56.734s
    Startup finished in 1.708s (kernel) + 11.141s (initrd) + 43.628s (userspace) = 56.478s

    # no stack protector
    Startup finished in 1.743s (kernel) + 11.190s (initrd) + 43.709s (userspace) = 56.643s
    Startup finished in 1.763s (kernel) + 11.216s (initrd) + 43.767s (userspace) = 56.747s
    Startup finished in 1.711s (kernel) + 11.283s (initrd) + 43.878s (userspace) = 56.873s

This patch introduces a configure option to disable the stack protector
entirely, and conditional stack protector flag selection (in order,
based on availability): -fstack-protector-strong, -fstack-protector-all,
no stack protector.

Signed-off-by: Steven Noonan <snoonan@amazon.com>
Cc: Anthony Liguori <aliguori@amazon.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
[Prefer -fstack-protector-all to -fstack-protector, suggested by
 Laurent Desnogues. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -198,6 +198,7 @@ audio_win_int=""
 cc_i386=i386-pc-linux-gnu-gcc
 libs_qga=""
 debug_info="yes"
+stack_protector=""
 
 # Don't accept a target_list environment variable.
 unset target_list
@@ -950,6 +951,10 @@ for opt do
   ;;
   --disable-werror) werror="no"
   ;;
+  --enable-stack-protector) stack_protector="yes"
+  ;;
+  --disable-stack-protector) stack_protector="no"
+  ;;
   --disable-curses) curses="no"
   ;;
   --enable-curses) curses="yes"
@@ -1219,6 +1224,7 @@ Advanced options (experts only):
   --disable-sparse         disable sparse checker (default)
   --disable-strip          disable stripping binaries
   --disable-werror         disable compilation abort on warning
+  --disable-stack-protector disable compiler-provided stack protection
   --disable-sdl            disable SDL
   --enable-sdl             enable SDL
   --with-sdlabi            select preferred SDL ABI 1.2 or 2.0
@@ -1439,9 +1445,15 @@ for flag in $gcc_flags; do
     fi
 done
 
-if compile_prog "-Werror -fstack-protector-all" "" ; then
-    QEMU_CFLAGS="$QEMU_CFLAGS -fstack-protector-all"
-    LIBTOOLFLAGS="$LIBTOOLFLAGS -Wc,-fstack-protector-all"
+if test "$stack_protector" != "no" ; then
+  gcc_flags="-fstack-protector-strong -fstack-protector-all"
+  for flag in $gcc_flags; do
+    if compile_prog "-Werror $flag" "" ; then
+      QEMU_CFLAGS="$QEMU_CFLAGS $flag"
+      LIBTOOLFLAGS="$LIBTOOLFLAGS -Wc,$flag"
+      break
+    fi
+  done
 fi
 
 # Workaround for http://gcc.gnu.org/PR55489.  Happens with -fPIE/-fPIC and