Summary
-------

Remove references and support for the `java.compiler` system property.

Problem
-------

`java.lang.Compiler` was an interface which was supposed to provide a common abstraction for the underlying JIT implementation. However,
given the wide variation of the underlying JIT implementations, this interface was never implemented or supported by the underlying 
JIT implementations. As such, this interface was deprecated for removal in Java 9 https://bugs.openjdk.org/browse/JDK-4285505.
Recently, this interface has now been completely removed https://bugs.openjdk.org/browse/JDK-8205129.

The `java.lang.Compiler` has a corresponding underspecified `java.compiler` system property. This is one of the system properties listed
in the API documentation of `System.getProperties()` and the description of this property states - "Name of JIT compiler to use".

Since the `java.lang.Compiler` interface was never implemented or supported by JIT implementations, there never was a specified set of values for this `java.compiler` system property. In hotspot implementation, the only exception to this were the values `NONE` and empty string. If this system property was set to a value of `NONE` or was set as empty string and if `-Xdebug` option wasn't specified while launching `java`, then the hotspot implementation would consider this as an instruction to run the application in interpreted-only mode. This would be equivalent to launching the application with the hotspot implementation specific `-Xint` option.

With the `java.lang.Compiler` interface now removed and the fact that this system property hasn't seen any practical use, removing this system property and its references would be a good idea.


Solution
--------

- Remove the system property "java.compiler".  This means removing the reference to this property from the table in `java.lang.System.getProperties()` API documentation and removing the hotspot implementation which had special treatment for the values `NONE` and empty string. 

- Change the hotspot implementation to log a warning message when it sees the use of this system property when launching `java`. We believe that removing the implementation in hotspot for `NONE` and empty string value for this property, when `-Xdebug` option isn't set, shouldn't cause major compatibility issue, because applications can regain interpreted-only mode by using the `-Xint` option instead.


Specification
-------------

    
    diff --git a/src/java.base/share/classes/java/lang/System.java b/src/java.base/share/classes/java/lang/System.java
    index f15b5861670..3fd13f89289 100644
    --- a/src/java.base/share/classes/java/lang/System.java
    +++ b/src/java.base/share/classes/java/lang/System.java
    @@ -760,8 +760,6 @@ public final class System {
          *     <td>List of paths to search when loading libraries</td></tr>
          * <tr><th scope="row">{@systemProperty java.io.tmpdir}</th>
          *     <td>Default temp file path</td></tr>
    -     * <tr><th scope="row">{@systemProperty java.compiler}</th>
    -     *     <td>Name of JIT compiler to use</td></tr>
          * <tr><th scope="row">{@systemProperty os.name}</th>
          *     <td>Operating system name</td></tr>
          * <tr><th scope="row">{@systemProperty os.arch}</th>


Launching `java` by passing  `-Djava.compiler` will generate a warning message:

    java -Djava.compiler=bar  Foo.java
    OpenJDK 64-Bit Server VM warning: The java.compiler system property is obsolete and no longer supported.


    java -Djava.compiler=NONE  Foo.java
    OpenJDK 64-Bit Server VM warning: The java.compiler system property is obsolete and no longer supported, use -Xint