Reduce default shard size.

Current HWASan build on master fails if OUT_DIR is ~17 characters,
passes with 3. Halve defaultShardSize to avoid this.

Bug: 170327170
Test: build with long OUT_DIR

Change-Id: I04e3cae91118891c0e28d895108670dd0d373a3f
diff --git a/genrule/genrule.go b/genrule/genrule.go
index e5ee3fc..5a9376b 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -703,7 +703,7 @@
 	Shard_size *int64
 }
 
-const defaultShardSize = 100
+const defaultShardSize = 50
 
 func NewGenRule() *Module {
 	properties := &genRuleProperties{}