Support installing headers for bootstrapping libgcc

[Roland McGrath]

> -$(objpfx)syscall-%.h $(objpfx)syscall-%.d: ../sysdeps/unix/sysv/linux/sys/syscall.h
> +$(objpfx)bits/syscall.h $(objpfx)bits/syscall.d: ../sysdeps/unix/sysv/linux/sys/syscall.h

The old pattern rule has the semantics that the rule is known to create
both targets.  Changing it to an explicit rule has the effect of defining a
separate rule for each target, so the commands may be run twice in
parallel.  A pattern (implicit) rule is the only way to tell make that one
set of commands produces two output files.  You can use a silly pattern
sure not to match anything else, e.g.:

$(objpfx)bits/syscall%h $(objpfx)bits/syscall%d: ...

With this fixed, the change is OK with me.


Thanks,
Roland