mips: Switch inferior function calls to ON_STACK method.

This patch switches the mips code to use the ON_STACK method
for function calls instead of AT_SYMBOL, which we want to remove.

gdb/ChangeLog:

        * mips-tdep.c (mips_push_dummy_code): New function.
        (mips_gdbarch_init): Set the gdbarch call_dummy_location to
        ON_STACK and install mips_push_dummy_code as our gdbarch
        push_dummy_code routine.