flow: exports_files for scripts/synth.tcl

Lets bazel-orfs reference `@orfs//flow:scripts/synth.tcl` as a direct
Label, so its `_synth_tcl` attr default can point at this script
instead of vendoring its own copy at the bazel-orfs repo root.

bazel-orfs has carried a forked synth.tcl since parallel partition
synthesis landed (e9b84aa orfs: ship parallel synthesis scripts and
wrapper Makefile, bazel-orfs). The fork has drifted on a couple of
points (missing the `-noabc` on `synth -run fine:`, lost
`SYNTH_MINIMUM_KEEP_SIZE` branch, etc.) and the drift has caused real
WNS regressions when yosys was bumped past 0.62. Exporting this label
upstream is the first step toward retiring the bazel-orfs copy and
letting it inherit fixes from this script directly.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/BUILD

@@ -18,6 +18,15 @@ exports_files(
     visibility = ["//visibility:public"],
 )
 
+# Expose synth.tcl as an addressable source label so bazel-orfs can
+# point its `_synth_tcl` attr default at `@orfs//flow:scripts/synth.tcl`
+# instead of vendoring its own (drifting) copy at the bazel-orfs repo
+# root. See bazel-orfs `private/rules.bzl`.
+exports_files(
+    ["scripts/synth.tcl"],
+    visibility = ["//visibility:public"],
+)
+
 # files shared between scripts/synth.sh and scripts/flow.sh steps
 MAKEFILE_SHARED = [
     "scripts/variables.json",