pinentry: link with libatomic when needed

On some architectures, atomic binutils are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

sparc-buildroot-linux-uclibc/sysroot/lib/libatomic.so.1: error adding symbols: DSO missing from command line

This is often for example the case on sparcv8 32 bit.

Fixes:

 http://autobuild.buildroot.net/results/fd6bee70ff20bee9607a9f6f557a3a793d00cd9f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: use LIBS instead of LDFLAGS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>