build: Link with libatomic if necessary

The implementation of atomic operations in gcc uses atomic opcodes if
supported, but falls back to a library implementation if necessary.
Link that library on architectures where 64-bit atomic operations need it,
such as Debian armel (ARMv5 EABI softfloat), mipsel and powerpc.

Resolves: https://gitlab.gnome.org/GNOME/gjs/-/issues/442


Signed-off-by: Simon McVittie <smcv@debian.org>