thrift: remove BR2_ARCH_HAS_ATOMICS dependency

thrift builds perfectly fine on SPARC, which doesn't implement any of
the __sync built-ins, so we can get rid of the BR2_ARCH_HAS_ATOMICS
dependency.

The dependency on atomic operations originated in commit
1aaa14d8 ("thrift: disable for ARC"),
but there was no reference to any autobuilder failure. Our testing
with an ARC toolchain that doesn't have atomics shows that thrift
builds just fine, and code inspection also shows no usage of
__sync_*() built-ins. At the time of the commit, the thrift version
was 0.9.1, which also did not contain any __sync_*() call.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: build-tested on an ARC toolchain without
 atomics, with CPU 770D]
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>