[build] Increment default thread stack size from to 2MB (was 1MB) (scala#23638)

I suspect the StackOverflow errors started to get triggered recently as
our build got larger/more complicated - increae in the thread stack size
should mitigate the issue, otherwise, we'll need to investigate it
again.

Author: WojciechMazur

.jvmopts

@@ -1,5 +1,5 @@
--Xss1m
--Xms512m
--Xmx4096m
+-Xss2m
+-Xms1024m
+-Xmx8192m
 -XX:MaxInlineLevel=35
 -XX:ReservedCodeCacheSize=512m