Add a -XX:+UseGraalJIT VM flag

[dougxc]

This change adds support for -XX:+UseGraalJIT as a macro VM flag for:

-XX:+UnlockExperimentalVMOptions
-XX:+EnableJVMCIProduct
-XX:+UseJVMCICompiler
-Djvmci.Compiler=graal
-XX:-UnlockExperimentalVMOptions

It allows for a customer to use a supported and branded Graal-compiler based JIT in a JDK product.

I've tested it with a drop-in copy of the libgraal library:

> cp libjvmcicompiler.dylib ~/jdk-jdk/open/build/macosx-aarch64/images/jdk/lib
> ~/jdk-jdk/open/build/macosx-aarch64/images/jdk/bin/java -XX:+UseGraalJIT -Dgraal.ShowConfiguration=info -cp ~/Dropbox/bin CountUppercase
[Use -Dgraal.LogFile=<path> to redirect Graal log output to a file.]
Using "Graal Community compiler with Truffle extensions" loaded from a Native Image shared library
1 (97 ms)
2 (92 ms)
3 (89 ms)
4 (82 ms)
5 (71 ms)
6 (72 ms)
7 (75 ms)
8 (69 ms)
9 (69 ms)
total: 0 (785 ms)

Removing the libgraal library provides a sensible error message:

> rm ~/jdk-jdk/open/build/macosx-aarch64/images/jdk/lib/libjvmcicompiler.dylib
> ~/jdk-jdk/open/build/macosx-aarch64/images/jdk/bin/java -XX:+UseGraalJIT -Dgraal.ShowConfiguration=info -cp ~/Dropbox/bin CountUppercase
JVMCI compiler 'graal' specified by jvmci.Compiler not found