RFR: 8256256: UL should not use heap allocation for output string

[Yumin Qi]

Hi, Please review
  Unified Logging uses 512 bytes buffer on stack for printing out message, above that will allocate heap for extra space needed. This may potentially may cause a circulation when we log the heap allocation and the log message is over 512. The max logging buffer size now is increased to 4096 and above that, output will be truncated.

Tests: tier1,tier4

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Commit messages:
 - 8256256: UL should not use heap allocation for output string

Changes: https://git.openjdk.java.net/jdk/pull/1246/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1246&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256256
  Stats: 29 lines in 3 files changed: 6 ins; 15 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1246/head:pull/1246

PR: https://git.openjdk.java.net/jdk/pull/1246