RFR: 8339507: Test generation tool and gtest for testing APX encoding of extended gpr instructions [v3]

[hanklo6]

> Add test generation tool and gtest for testing APX encoding of instructions with extended general-purpose registers.
> 
> Intel Advanced Performance Extensions (APX) doubles the number of general-purpose registers, from 16 to 32. For more information about APX, see https://www.intel.com/content/www/us/en/developer/articles/technical/advanced-performance-extensions-apx.html.
> 
> By specification, instruction encoding remains unchanged for instructions using only the lower 16 GPRs. For cases where one or more instruction operands reference extended GPRs (Egprs), encoding targets either REX2, an extension of REX encoding, or an extended version of EVEX encoding. These new encoding schemes extend or modify existing instruction prefixes only when Egprs are used.
> 
> ### Generate test instructions
> With `binutils = 2.43`
> * `python3 x86-asmtest.py > asmtest.out.h`
> ### Run test
> *  `make test TEST="gtest:AssemblerX86"`

hanklo6 has updated the pull request incrementally with one additional commit since the last revision:

  Add comment and defined

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20857/files
  - new: https://git.openjdk.org/jdk/pull/20857/files/766582d8..e6b4abd8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20857&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20857&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20857.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20857/head:pull/20857

PR: https://git.openjdk.org/jdk/pull/20857