vulkan: Fix weak symbol emulation when compiling with MSVC

vulkan: Fix weak symbol emulation when compiling with MSVC

Mapping unimplemented entrypoints to a global function pointer variable
initialized to NULL is a bit cumbersome, and actually led to a bug
in the vk_xxx_dispatch_table_from_entrypoints() template: the !override
case didn't have the right check on the source table entries. Instead of
fixing that case, let's simplify the logic by creating a stub function
and making the alternatename pragma point to this stub. This way we get
rid of all those uneeded xxx_Null symbols/variables and simplify the
tests in vk_xxxx_dispatch_table_from_entrypoints().

Cc: mesa-stable
Fixes: 98c622a96e28 ("vulkan: Update dispatch table gen for Windows")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13348>