[gdb] Update syscalls/{amd64,i386}-linux.xml

[gdb] Update syscalls/{amd64,i386}-linux.xml

- Add a script syscalls/gen-header.py, based on syscalls/arm-linux.py.
- Add a script syscalls/update-linux.sh (alongside update-freebsd.sh and
  update-netbsd.sh).
- Use syscalls/update-linux.sh to update syscalls/{amd64,i386}-linux.xml.in.
- Regenerate syscalls/{amd64,i386}-linux.xml using syscalls/Makefile.

In gdb/syscalls/i386-linux.xml.in, updating has the following notable effect:
...
-  <syscall name="madvise1" number="220"/>
-  <syscall name="getdents64" number="221"/>
-  <syscall name="fcntl64" number="222"/>
+  <syscall name="getdents64" number="220"/>
+  <syscall name="fcntl64" number="221"/>
...

I've verified in ./arch/x86/entry/syscalls/syscall_32.tbl that the numbers are
correct.

Tested on x86_64-linux.