RFR: 8138916: Logging write function does not allow for long enough messages

[Rachel Protacio]

Hello, all,

Please review this fix to the faulty write function from the Unified 
Logging framework.

Summary: In logging/log.hpp, the logging vwrite function previously 
asserted that the buffer remains within 512 characters, which is too 
short for logging message of non-pre-determined length, e.g. for vtable 
and itable function names. Now, the function resizes the buffer to the 
appropriate length, asserting only that the resulting buffer is valid. 
Includes tag "logtest" to test that logging can print an output of 518 
characters.

Open webrev: http://cr.openjdk.java.net/~rprotacio/8138916/
Bug: https://bugs.openjdk.java.net/browse/JDK-8138916

Includes own acceptance test, and passes JPRT and remote hotspot/test tests.

Thank you,
Rachel