mksigtab.sh: recurse once when adding signals to SIGLIST

mksigtab.sh: recurse once when adding signals to SIGLIST

    On MIPS, SIGABRT is defined like this:
     #define SIGIOT   6
     #define SIGABRT  SIGIOT

    This breaks addsig which tries to append __SIGIOT_ to SIGLIST. Signal
    number 6 is later added to the output and go complains about a
    duplicate signal number.

    Fix by recursing once when obtaining the signal number from
    gen-sysinfo.go if the signal is defined as an alias of another signal.
    Also modify the sed expression to 's/.* = //' which is equivalent to
    the original expression but is less misleading given that it might not
    match a number.

    Reviewed-on: https://go-review.googlesource.com/43252

From-SVN: r247948