Bug 1692940 - Switch aom build to nasm instead of yasm. r=firefox-build-system-reviewers,jbauman,dmajor,andi

nasm doesn't like compiling x86_abi_support.asm
(https://bugzilla.nasm.us/show_bug.cgi?id=3392738), which is actually an
include file, rather than a source file, so it shouldn't have been in
the list of sources in the first place (libvpx has a similar file that
is excluded already, for instance).

I was considering updating the vendoring script, but it turns out it
doesn't produce the current contents in-tree (which even breaks the
build), and aom is set to be removed (bug 1635296)...

Differential Revision: https://phabricator.services.mozilla.com/D105430