[gdb/syscalls] Use update-linux-from-src.sh for arm-linux

[gdb/syscalls] Use update-linux-from-src.sh for arm-linux

I tried to use arm-linux.py to regenerate arm-linux.xml.in, but it didn't work.

Fix this by:
- adding handling of arm-linux.xml.in in update-linux-from-src.sh,
- regenerating arm-linux.xml.in using update-linux-from-src.sh and linux 6.11
  sources,
- regenerating arm-linux.xml using make, and
- removing arm-linux.py.

This changes the name "oldolduname" into "olduname".

Tested on arm-linux.  Verified with shellcheck.