MINOR: double -Xss setting from 2m to 4m in build.gradle (#8264)

I have seen an increased incidence in StackOverflowError(s) when compiling scala. This
change doubles the max stack size to 4m.

```
> Task :core:compileScala FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':core:compileScala'.
> java.lang.StackOverflowError (no error message)
```

Reviewers: Andrew Choi <a24choi@edu.uwaterloo.ca>, Ismael Juma <ismael@juma.me.uk>

build.gradle

@@ -469,7 +469,7 @@ subprojects {
 
     configure(scalaCompileOptions.forkOptions) {
       memoryMaximumSize = '1g'
-      jvmArgs = ['-Xss2m']
+      jvmArgs = ['-Xss4m']
     }
   }