As part of the migration away from obsolete binutils we want to retire GNU as. Most assembly files used on amd64 have a .S extension and are assembled with Clang's integrated assembler. We also have a rule for .asm files, which pass -x assembler-with-cpp to the Clang driver.
The Linuxulator has bespoke rules for invoking the compiler driver to assemble .s files and was already using Clang's IAS, but rename the assembly files for consistency.