package/libatomic_ops: fix compilation fail on sparc

Fixes http://autobuild.buildroot.net/results/6d8/6d83fa5d69572cec5c96be4b7651f9b113a1a19c/

libatomic_ops by default requires SPARC v9. buildroot's two supported
sparc arches (SPARCv8, and leon3) are both SPARCv8-based. Unfortunately
libatomic_ops's support for SPARCv8 is incomplete.

The library includes fallbacks but these must expressly be enabled by
defining a macro, enabled by this patch. Note that I'm testing for the
SPARC variants rather than BR2_sparc, in case someone implements SPARCv9
support in the future.

Discussion of this workaround described by the maintainer here :
https://github.com/ivmai/libatomic_ops/issues/9



Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>