squid: rework atomic handling

squid can use the __sync built-ins when available, but uses an
AC_TRY_RUN autoconf tests to check their availability, which isn't
compatible with cross-compilation. Due to this, squid.mk is already
hinting the configure script about this by passing
squid_cv_gnu_atomics=yes/no depending on the availability of atomic
operations.

So far, squid.mk was assuming that BR2_ARCH_HAS_ATOMICS &&
BR2_ARCH_IS_64 was needed, since 8 bytes __sync built-ins are
used. However, this was a bit too restrictive, since certain 32 bits
architectures (ARM, x86) do provide 8 bytes __sync built-ins.

So, instead of using BR2_ARCH_HAS_ATOMICS, we now rely on
BR2_TOOLCHAIN_HAS_SYNC_4 and BR2_TOOLCHAIN_HAS_SYNC_8, since both 4
bytes and 8 bytes __sync built-ins are tested by the autoconf test.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>