MFC r339624:

Remove the need for backslashes in syscalls.master.

Join non-special lines together until we hit a line containing a '}'
character. This allows the function declaration body to be split
across multiple lines without backslash continuation characters.

Continue to join lines ending with backslashes to allow gradual
migration and to support out-of-tree syscall vectors

Reviewed by: emaste, kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17488