[jdk11u-dev] RFR: 8267304: Bump global JTReg memory limit to 768m

[Aleksey Shipilev]

Clean backport to improve testing speed.

Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`

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Commit messages:
 - Backport e749f75de015fd9ef47f7a962f232e2ea677aba5

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/575/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=575&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/jdk11u-dev/pull/575.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/575/head:pull/575

PR: https://git.openjdk.java.net/jdk11u-dev/pull/575