arm: Enable USE_ATOMIC_COMPILER_BUILTINS (BZ #24774)

arm: Enable USE_ATOMIC_COMPILER_BUILTINS (BZ #24774)

As per other architectures.  I have checked on a armv8 hardware with
the following configurations:

  arm-linux-gnueabihf (gcc built with --with-float=hard --with-cpu=arm926ej-s)
  armv5-linux-gnueabihf (-march=armv5te -mfpu=vfpv3)
  armv7-linux-gnueabihf (-march=armv7-a -mfpu=vfpv3)
  armv7-thumb-linux-gnueabihf (-march=armv7-a -mfpu=vfpv3 -mthumb)
  armv7-neon-linux-gnueabihf (-march=armv7-a -mfpu=neon)
  armv7-neonhard-linux-gnueabihf (-march=armv7-a -mfpu=neon -mfloat-abi=hard)

Without any regression.

I haven't dig into the code, but since Linux atomic-machine.h handle
pre-ARMv6 and ARMv6 I expect the compiler might have some small room
to optimize.

The code size also improves is most of the configurations:

* master

   text    data     bss     dec     hex filename
1727801    9720   37928 1775449  1b1759	 arm-linux-gnueabihf/libc.so
1691729    9720   37928 1739377  1a8a71	 arm-linux-gnueabihf-armv7-disable-multi-arch/libc.so
1725509    9720   37928 1773157  1b0e65	 armv5-linux-gnueabihf/libc.so
1700757    9720   37928 1748405  1aadb5	 armv6-linux-gnueabihf/libc.so
1698973    9720   37928 1746621  1aa6bd	 armv6t2-linux-gnueabihf/libc.so
1695481    9752   37928 1743161  1a9939	 armv7-linux-gnueabihf/libc.so
1692917    9744   37928 1740589  1a8f2d	 armv7-neonhard-linux-gnueabihf/libc.so
1692917    9744   37928 1740589  1a8f2d	 armv7-neon-linux-gnueabihf/libc.so
1225353    9752   37928 1273033  136cc9	 armv7-thumb-linux-gnueabihf/libc.so

* patched

   text    data     bss     dec     hex filename
1726805    9720   37928 1774453  1b1375 arm-linux-gnueabihf/libc.so
1689321    9720   37928 1736969  1a8109	arm-linux-gnueabihf-armv7-disable-multi-arch/libc.so
1724433    9720   37928 1772081  1b0a31 armv5-linux-gnueabihf/libc.so
1698301    9720   37928 1745949  1aa41d armv6-linux-gnueabihf/libc.so
1696525    9720   37928 1744173  1a9d2d armv6t2-linux-gnueabihf/libc.so
1693009    9752   37928 1740689  1a8f91 armv7-linux-gnueabihf/libc.so
1690493    9744   37928 1738165  1a85b5 armv7-neonhard-linux-gnueabihf/libc.so
1690493    9744   37928 1738165  1a85b5 armv7-neon-linux-gnueabihf/libc.so
1223837    9752   37928 1271517  1366dd armv7-thumb-linux-gnueabihf/libc.so

The idea is eventually move all architectures to use compiler builtins.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>