RFR: 8293019: [JVMCI] change ratio of libgraal to C1 threads and use one isolate per libgraal thread

[Doug Simon]

[JDK-8242440](https://bugs.openjdk.org/browse/JDK-8242440) allows the VM to use one isolate per libgraal thread (OIPLT). This has a number of benefits such better isolation of compiler crashes and avoids GC interference between isolates. Experimentation has shown that reducing the ratio of libgraal threads to C1 threads also helps keep the max RSS down. This PR makes OIPLT the default and adds a `JVMCINativeLibraryThreadFraction` flag for configuring the libgraal to C1 thread ratio. The default value for `JVMCINativeLibraryThreadFraction` is `0.33` which means there are 2 C1 threads per libgraal thread.

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Commit messages:
 - set JVMCIThreadsPerNativeLibraryRuntime default to 1
 - add JVMCINativeLibraryThreadFraction to configure ratio of libjvmci to c1 threads

Changes: https://git.openjdk.org/jdk/pull/10094/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10094&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293019
  Stats: 23 lines in 3 files changed: 18 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10094.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10094/head:pull/10094

PR: https://git.openjdk.org/jdk/pull/10094