anv: Disable __gen_validate_value if NDEBUG is set.

anv: Disable __gen_validate_value if NDEBUG is set.

We were enabling undefined memory checking for genxml values based on
Valgrind being installed at build time, even for release builds.  This
generates piles and piles of assembly whenever you touch genxml.

With gcc 7.3.1 and -O3 and -march=native on a Kabylake with Valgrind
installed at build time:

      text    data    bss     dec    hex filename
   5978385  262884  13488 6254757 5f70a5 libvulkan_intel.so
   3799377  262884  13488 4075749 3e30e5 libvulkan_intel.so

That's a 36% reduction in text size.

Fixes: 047ed02723071d7eccbed3210b5be6ae73603a53 (vk/emit: Use valgrind to validate every packed field)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>