add module_x86_64only product

64bit only systems won't load modules with mixed 32/64 binaries. We need
a 64bit only product to build with.

Bug: 205990590
Test: lunch module_x86_64only && m
Test: vendor/google/build/build_mainline_modules.sh
Change-Id: I906f6cd10882986c8182ed16916e07c0f0d8b32e