Code changes required after JDK-8291473

[Filip Krakowski]

Hi,

since jextract does not work with the latest changes introduced in 
https://git.openjdk.org/panama-foreign/pull/694, I started to work on 
adjusting the sources in [1]. I took the following steps so far:

   1. Find & replace all occurrences and imports of "Addressable" and 
"MemoryAddress" and replace it with "MemorySegment"
   2. Change all occurences of "Linker.downcallType" and 
"Linker.upcallType" with "Linker.methodType"
   3. Remove duplicate if-statements resulting from Step 1
   4. Make C_POINTER$LAYOUT unbounded (in 
org.openjdk.jextract.impl.ToplevelBuilder#primitiveLayoutString)
   5. Use MemorySegment#equals instead of reference comparison inside 
org.openjdk.jextract.clang.org.openjdk.jextract.clang#getLocation
   6. Tweak build.gradle so that the user does not have to specify the 
jdk home directory. Gradle will automatically search for the specified 
JDK version. It can also be instructed to search inside custom locations 
using the org.gradle.java.installations.paths property, which can be set 
inside ${HOME}/.gradle/gradle.properties.

Since my approach is relatively naive ("find & replace" + fixing 
compiler and runtime errors), important parts of the code may be 
incorrect at the moment. I tested it with my Gradle plugin's demo 
project [2], which works. But again, this is only a simple program just 
calling printf and doing nothing fancy. In the next step I will try to 
rewrite some jtreg tests and see if they pass. Hopefully, this should 
point out all incorrect parts.

If you want to try it out, you need to compile [1] and add the resulting 
"bin"-Folder ("$PROJECT_DIR/build/jextract/bin") to your PATH. 
Afterwards [2] should just work by executing "./gradlew run". If 
successful, the program should print "Hello World" (using the native 
printf function).

I would greatly appreciate feedback regarding my general approach to 
code changes. In particular, I am unsure about changing the C_POINTER 
layout to be unbounded.

Best regards,
Filip

[1] - https://github.com/krakowski/jextract/tree/jdk20
[2] - https://github.com/krakowski/jextract-demo