m68k: Ignore disabled HSYNC interrupt on Atari for irqs_disabled()

When running a multi-platform kernel on Atari, warning messages like
the following may be printed:

    WARNING: at /root/linux-3.10.1/init/main.c:698 do_one_initcall+0x12e/0x13a()
    initcall param_sysfs_init+0x0/0x1a4 returned with disabled interrupts

This is caused by the different definitions of ALLOWINT for Atari and
other platforms:

    #if defined(MACH_ATARI_ONLY)
    #define ALLOWINT        (~0x500)
    #else
    #define ALLOWINT        (~0x700)
    #endif

On Atari, we want to disable the high-frequency HSYNC interrupt:
  - On Atari-only kernels, this is handled completely through ALLOWINT,
  - On multi-platform kernels, this is handled by disabling the HSYNC
    interrupt from the interrupt handler.

However, as in the latter case arch_irqs_disabled_flags() didn't ignore the
disabling of the HSYNC interrupt, irqs_disabled() would detect false
positives.

Ignore the HSYNC interrupt when running on Atari to fix this.
For single-platform kernels this test is optimized away by the compiler.

Reported-by: Thorsten Glaser <tg@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Thorsten Glaser <tg@debian.org>