Remove endbr64 instructions from the .s file with sfix
When GCC went from version 8 to version 9, it started inserted endbr64 instructions into the generated .s code. endbr64 is a signal that the address is a valid target for an indirect jump. Code thus annotated is more resistant to hacking and errors. But it messes up our ability to trick GDB into showing us good line numbers when stepping into subroutines, so I modified SFIX to just strip them out.