RFR: 8346278: Clean up some flag handing in flags-cflags.m4

[Erik Joelsson]

On Mon, 16 Dec 2024 12:31:04 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> WARNING_CFLAGS_JVM should not be used to set arbitrary warning flags for Hotspot.
> 
> We do not need to check for gcc 6+ anymore.

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/22762#pullrequestreview-2506519149