Meson requires a cross file for compiling i386 on x86_64. This
unfortunately needs to be generated on the fly to honor oss-fuzz'
compiler and compiler flags.
Supercedes #2823.
dav1d renamed the options build_asm and build_tools. build_asm=false
is no longer needed since the upstream fuzzer target disables the asm
code path when it runs under memory sanitizer.
Use enable_tools=false to skip building the dav1d CLI tools.
Fixes build failure caused by unavailable https://www.nasm.us/
The nasm version in ubuntu xenial is too old. The package from bionic
is new enough and usable without other dependencies from bionic.
Janne Grunau