RFR: 8267304: Bump global JTReg memory limit to 768m

[Aleksey Shipilev]

See the rationale in the RFE.

Additional testing:
 - [x] Linux x86_64 fastdebug `java/time/format/TestZoneTextPrinterParser.java` with aggressive GC options
 - [x] Linux x86_64 fastdebug `jdk:tier1`
 - [x] Linux x86_64 fastdebug `tier1`
 - [x] Linux x86_64 fastdebug `tier2`
 - [x] Linux x86_32 fastdebug `tier1`
 - [x] Linux x86_32 fastdebug `tier2`

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Commit messages:
 - 8267304: Bump global JTReg memory limit to 768m

Changes: https://git.openjdk.java.net/jdk/pull/4087/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4087&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267304
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4087.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4087/head:pull/4087

PR: https://git.openjdk.java.net/jdk/pull/4087