RFR (XS) 8155090: String concatenation fails with a custom SecurityManager that uses concatenation

[Aleksey Shipilev]

Hi,

Please review the fix for a shady bootstrapping issue, when a custom
SecurityManager is using string concatenation:
  https://bugs.openjdk.java.net/browse/JDK-8155090

The essence of the issue is that during StringConcatFactory::<clinit>,
we are reading the system properties via the privileged calls. When
user SecurityManager that uses string concatenation is set, we are
trying to produce a string concatenation stub in order to proceed, and
double-back on SCF. There, we try to run SCF methods without fully
complete <clinit>: the existing test fails with uninitialized static
final Strategy field.

The cleanest (yet subtle) solution here is to make sure the default SCF
settings are good to run with, which allows transient <clinit>
operations to complete normally:
  http://cr.openjdk.java.net/~shade/8155090/webrev.00/

Testing: offending test; java/lang/String jtregs

Thanks,
-Aleksey