sram: mix .lib and .lef files

[oharboe]

@hovind Failing example of trying to mix and match .lib and .lef files:

$ bazel build //sram:top_mix_synth
INFO: Invocation ID: a5629799-b434-4c4b-b0c8-91ead8eec75f
INFO: Analyzed target //sram:top_mix_synth (0 packages loaded, 0 targets configured).
ERROR: /home/oyvind/bazel-orfs/sram/BUILD:69:10: Action sram/results/asap7/top/mix/1_synth.rtlil failed: missing input file '//sram:results/asap7/sdq_17x64/base/sdq_17x64.lef'
ERROR: /home/oyvind/bazel-orfs/sram/BUILD:69:10: Action sram/results/asap7/top/mix/1_synth.rtlil failed: 1 input file(s) do not exist
Target //sram:top_mix_synth failed to build
Use --verbose_failures to see the command lines of failed build steps.
ERROR: /home/oyvind/bazel-orfs/sram/BUILD:69:10 Action sram/results/asap7/top/mix/1_synth.rtlil failed: 1 input file(s) do not exist
INFO: Elapsed time: 0.113s, Critical Path: 0.00s
INFO: 2 processes: 2 internal.
ERROR: Build did NOT complete successfully