build-many-glibcs.py: Implement update-syscalls command

This command uses pre-built compilers to re-install the Linux headers
from the current sources into a temporary location and runs glibc's
“make update-syscalls-lists” against that.  This updates the glibc
source tree with the current system call numbers.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>