anv: Define trampolines as the weak functions

anv: Define trampolines as the weak functions

Instead of having weak references to the anv functions and separate
trampoline functions with their own dispatch table, just make the
trampoline functions weak.  This gets rid of a dispatch table and
potentially lets the compiler delete the unused weak function.  The
end result is a reduction in the .text section of 5.7K and a reduction
in the .data section of 1.4K.

Before:

   text	   data	    bss	    dec	    hex	filename
3190329	 282232	   8960	3481521	 351fb1	_install/lib64/libvulkan_intel.so

After:

   text	   data	    bss	    dec	    hex	filename
3184548	 280792	   8960	3474300	 35037c	_install/lib64/libvulkan_intel.so

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>