Add support for strsignal, for platforms which have it but don't have sys_siglist (like Solaris 2.7.)

Add support for strsignal, for platforms which have it but don't have
sys_siglist (like Solaris 2.7.)
        * acconfig.h (NEED_DECLARATION_STRSIGNAL): Provide a stub.
        * collect2.c: Don't declare `sys_siglist' here.
        (my_strsignal): Prototype and define new function.  Use it in
        place of `sys_siglist' hacks.
        * mips_tfile.c:  Likewise.
        * configure.in (AC_CHECK_FUNCS): Check for strsignal.
        (GCC_NEED_DECLARATIONS): Likewise.
        * system.h (strsignal): Prototype it, if necessary.
        (sys_siglist): Declare it, if necessary.

From-SVN: r22403