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 (IAS); rename pxetram.s to
.S to use IAS as well.
The generated .text is identical (the entire .o file is not, as Clang
adds debug info.)
PR: 205250, 233094
Sponsored by: The FreeBSD Foundation
