tor: link with libatomic when needed

The following error occurs on architectures required to use gcc
libatomic for atomic intrinsics:

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

This is for example the case on sparcv8 32 bit.

Fixes:

 http://autobuild.buildroot.net/results/0f7107d0b703223f567c6749722bc82c81a04d54



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