[gdb/syscalls] Improve update-linux-from-src.sh

Some improvements in gdb/syscalls/update-linux-from-src.sh:
- use bash instead of sh
- use local to distinguish between local and global vars
  (which brings to light that pre uses the global rather than the local
  start_date)
- factor out main and parse_args
- factor out regen
- iterate over *.xml.in instead of *.in

Tested on aarch64-linux.  Verified with shellcheck.