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

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

Add a new script gdb/syscalls/update-linux-from-src.sh, that can be used to
generate *-linux.xml.in files from linux kernel sources, like so:
...
$ ./update-linux-from-src.sh ~/upstream/linux-stable.git
Skipping aarch64-linux.xml.in, no syscall.tbl
Generating amd64-linux.xml.in
Skipping arm-linux.xml.in, use arm-linux.py instead
Skipping bfin-linux.xml.in, no longer supported
Generating i386-linux.xml.in
Generating mips-n32-linux.xml.in
Generating mips-n64-linux.xml.in
Generating mips-o32-linux.xml.in
Generating ppc64-linux.xml.in
Generating ppc-linux.xml.in
Generating s390-linux.xml.in
Generating s390x-linux.xml.in
Generating sparc64-linux.xml.in
Generating sparc-linux.xml.in
...

Update *-linux.xml.in and *-linux.xml using linux kernel tag v5.18-rc6.