libatomic_ops: bump to version 7.2

The 1.2 we were using doesn't build on ARM Thumb platforms:

  http://autobuild.buildroot.org/results/94ef6be7dcb31021462e7313724217627d4b29df/build-end.log



Moreover, the 7.2 version is the one used by Debian/Ubuntu, and it has
been tested to build fine with the two packages that depend on
libatomic_ops: libdrm and pulseaudio.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>