RFR: 8327701: Remove the xlc toolchain [v2]

[Magnus Ihse Bursie]

On Fri, 8 Mar 2024 15:44:48 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> As of [JDK-8325880](https://bugs.openjdk.org/browse/JDK-8325880), building the JDK requires version 17 of IBM Open XL C/C++ (xlc). This is in effect clang by another name, and it uses the clang toolchain in the JDK build. Thus the old xlc toolchain is no longer supported, and should be removed.
>
> Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert SEARCH_PATH changes

make/autoconf/toolchain.m4 line 444:

> 442:   COMPILER_NAME=$2
> 443:   SEARCH_LIST="$3"
> 444:   SEARCH_PATH="$PATH"

I am note 100% sure about this; I think the intention of some of the old code amounted to this, but it was not entirely clear.

But, then again, I think this is a good idea, and I think we should do this on *all* platforms -- search the toolchain path before the normal path.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18172#discussion_r1517884839