bdwgc: update bdw-gc.pc to fix link with guile

When bdw-gc is linked with libatomic_ops, bdw-gc.pc must contain the
needed libraries (such as -latomic_ops) otherwise build of applications
such as guile will fail on link stage:

.libs/libguile_2.0_la-posix.o: In function `scm_tmpnam':
posix.c:(.text+0x2080): warning: the use of `tmpnam' is dangerous, better use `mkstemp'
  CCLD     guile
/home/buildroot/autobuild/run/instance-2/output/host/arm-buildroot-linux-gnueabi/sysroot/usr/lib/libgc.so: undefined reference to `AO_fetch_compare_and_swap_emulation'
/home/buildroot/autobuild/run/instance-2/output/host/arm-buildroot-linux-gnueabi/sysroot/usr/lib/libgc.so: undefined reference to `AO_store_full_emulation'

So set ATOMIC_OPS_LIBS to -latomic_ops when a system atomic_ops library
is used and use ATOMIC_OPS_LIBS in bdw-gc.pc.in

Fixes:
 - http://autobuild.buildroot.org/results/2b23d445e57a5e0f417f5cb9417b0a668bb7bf1c



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>