Rust: In 'type_for_mode' langhook also consider all 'int_n' modes/types

As reported in "Rust related issues" at the end of
<https://inbox.sourceware.org/gcc-patches/20221219212344.ewtuzd2jtztm77x2@lug-owl.de>
"Modula-2 / Rust: Many targets failing",
<Rust-GCC/gccrs#1713>
"Test failure on msp430-elfbare target", for '--target=msp430-elfbar' we ICE:

    <built-in>: internal compiler error: Segmentation fault
    0xf2efbf crash_signal
          ../../gcc/gcc/toplev.cc:314
    0x120c8c7 build_function_type(tree_node*, tree_node*, bool)
          ../../gcc/gcc/tree.cc:7360
    0x120cc20 build_function_type_list(tree_node*, ...)
          ../../gcc/gcc/tree.cc:7442
    0x120d16b build_common_builtin_nodes()
          ../../gcc/gcc/tree.cc:9883
    0x8449b4 grs_langhook_init
          ../../gcc/gcc/rust/rust-lang.cc:132
    [...]

This is due to 'tmp == NULL' for 'gcc/tree.cc:build_common_builtin_nodes':

    tmp = lang_hooks.types.type_for_mode (targetm.eh_return_filter_mode (), 0);

..., were 'targetm.eh_return_filter_mode' (that is,
'gcc/targhooks.cc:default_eh_return_filter_mode',
via calling 'targetm.unwind_word_mode':
'gcc/config/msp430/msp430.cc:msp430_unwind_word_mode') returns 'PSImode',
and 'lang_hooks.types.type_for_mode' (that is,
'gcc/rust/rust-lang.cc:grs_langhook_type_for_mode') returns 'NULL' for
'PSImode'.

As, for example, discussed in <https://gcc.gnu.org/PR46805>
"ICE: SIGSEGV in optab_for_tree_code (optabs.c:407) with -O -fno-tree-scev-cprop -ftree-vectorize",
we have to support "random" modes/types in the 'type_for_mode' langhook.

	gcc/rust/
	* rust-lang.cc (grs_langhook_type_for_mode): Also consider all
	'int_n' modes/types.