RFR: 8277385: Zero: Enable CompactStrings support

[Aleksey Shipilev]

This enables `CompactStrings` for Zero. When we were doing original Compact Strings in JDK 9, we disabled the support on non-primary platforms, hoping relevant maintainers would follow up with platform-specific work. Here is me following up, as Zero maintainer :) There is little to do on Zero side, as it is pure interpreter without String intrinsics. There is still benefit of doing less work with smaller Strings.

There are no regressions on the benchmarks I tried, and some benchmarks improve significantly. Notably, specjvm:{compiler,sunflow,derby,xmlvalidation} improve about 5%, specjvm:{serial,xmltransform} improve about 20% on x86_64.

Additional testing:
 - [x] Linux x86_64 Zero benchmarks
 - [ ] Linux x86_64 Zero `tier1`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/6459/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6459&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277385
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6459.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6459/head:pull/6459

PR: https://git.openjdk.java.net/jdk/pull/6459