stand/i386/libi386: rename .s file to .S to use Clang IAS

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