The way ATTR_WEAK works is changed to eliminate the "don't declare as
weak on MinGW" weirdness. When a weak is not undefined, MinGW requires
the definition to be a regular symbol. Instead of declaring as weak
everywhere, we now declare the symbol as a regular by default, and only
make it weak when it needs to fallback to NULL when undefined, which is
only needed for the dispatch table. This unifies the approach for Unix
and MinGW.
The name ATTR_WEAK is changed to VK_ENTRY_WEAK since it's now controlled
by the entrypoint specific VK_ENTRY_USE_WEAK flag.
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29986>
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