sysdeps/init_array: Add PREINIT_FUNCTION to crti.S

sysdeps/init_array: Add PREINIT_FUNCTION to crti.S

The RISC-V port contains a crti.S that simply contains a link to
PREINIT_FUNCTION (when defined).  As this should be entirely generic,
Joseph Myers suggested that we update the generic init_array version to
contain this.  Since RISC-V is the only user of init_array this won't
break any existing ports.

2018-01-29  Palmer Dabbelt  <palmer@sifive.com>

        * sysdeps/init_array/crti.S (.section .init_array): Add
        PREINIT_FUNCTION when defined.