RFR: 8253757: Add LLVM-based backend for hsdis [v6]

[Magnus Ihse Bursie]

> This patch expands the newly added system for hsdis backends to include LLVM.
> 
> The actual code in hsdis-llvm.cpp is based heavily on the work by @luhenry, as published in the never integrated PR https://github.com/openjdk/jdk/pull/392. (I have basically just ripped out the binutils-based part of it.)
> 
> Unfortunately I have not been able to make this work properly on Windows. With some additional flags I made it compile without complaints, but it caused hotspot to segfault in `LoadLibrary` (!) in `os::dll_load` when I tried to load the library. This is somewhat ironic, since the initial implementation was created by Ludovic for the very purpose of using it on Windows.
> 
> The lack of Windows support in this patch does not mean it is impossible to get it to work, just that I need to co-operate with someone who has more experience of compiling LLVM on Windows, and/or are more eager to get this combination to work.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Restructure README to prepare for section on LLVM

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5920/files
  - new: https://git.openjdk.java.net/jdk/pull/5920/files/a4d9896e..47cf9a1b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5920&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5920&range=04-05

  Stats: 58 lines in 1 file changed: 30 ins; 22 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5920.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5920/head:pull/5920

PR: https://git.openjdk.java.net/jdk/pull/5920