package/ltp-testsuite: refine architecture dependency

package/ltp-testsuite: refine architecture dependency

As stated in commit [1], sparc toolchains doesn't have
any of __sync_*() family of functions implementation.

When __sync_add_and_fetch() is missing, ltp fallback to a local
implementation of tst_atomic_add_return() specific for each
supported architecture.
But there is none for sparc.

So add a dependency on BR2_TOOLCHAIN_HAS_SYNC_4 except for
architectures where a specific implementation is provided
in ltp-testsuite.

Fixes:
http://autobuild.buildroot.net/results/d7c/d7c3b145a64ed3916b89ddb4090050f3b9205e37

[1] 6856e417da4f3aa77e2a814db2a89429af072f7d

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>