jit: fix __builtin_unreachable [PR 95426]

jit: fix __builtin_unreachable [PR 95426]

PR jit/95426 reports a crash deep inside "expand" when using
__builtin_unreachable via gcc_jit_context_get_builtin_function,
due to BLOCK_FOR_INSN being erroneously used on a barrier within
rtl_verify_bb_pointers.

The root cause turns out to be that I didn't implement
LANG_HOOKS_COMMON_ATTRIBUTE_TABLE and LANG_HOOKS_FORMAT_ATTRIBUTE_TABLE
for the jit "frontend".  When building a decl for the builtin, the
libgccjit frontend generates a chain of attributes names, but when
this is passed to decl_attributes and the attributes are looked up by
namespace and name within lookup_scoped_attribute_spec, attributes_table
is empty.  Hence no attributes were being associated with the fndecl,
and so ECF_NORETURN was not set on the gimple_call (along with various
other flags missing on the decl, etc), and so the call is treated as
not terminating its BB, and so the CFG rapidly diverges from the
equivalent created by the C frontend.

This patch fixes things by implementing these langhooks, copying the
minimal attribute-handling code from LTO.  I stepped through the
creation of the fndecl and verified that with this fix it has the same
attributes as the equivalent created by the C frontend.

gcc/jit/ChangeLog:
	PR jit/95426
	* dummy-frontend.c: Include "options.h", "stringpool.h", and
	"attribs.h".
	(ATTR_EXCL): New, copied from lto/lto-lang.c.
	(attr_noreturn_exclusions): Likewise.
	(attr_returns_twice_exclusions): Likewise.
	(attr_const_pure_exclusions): Likewise.
	(jit_attribute_table): Likewise, copied from lto_attribute_table.
	(jit_format_attribute_table): Likewise, copied from
	lto_format_attribute_table.
	(handle_noreturn_attribute): New, copied from lto/lto-lang.c.
	(handle_leaf_attribute): Likewise.
	(handle_const_attribute): Likewise.
	(handle_malloc_attribute): Likewise.
	(handle_pure_attribute): Likewise.
	(handle_novops_attribute): Likewise.
	(get_nonnull_operand): Likewise.
	(handle_nonnull_attribute): Likewise.
	(handle_nothrow_attribute): Likewise.
	(handle_sentinel_attribute): Likewise.
	(handle_type_generic_attribute): Likewise.
	(handle_transaction_pure_attribute): Likewise.
	(handle_returns_twice_attribute): Likewise.
	(handle_patchable_function_entry_attribute): Likewise.
	(ignore_attribute): Likewise.
	(handle_format_attribute): Likewise.
	(handle_format_arg_attribute): Likewise.
	(handle_fnspec_attribute): Likewise.
	(LANG_HOOKS_COMMON_ATTRIBUTE_TABLE): Define.
	(LANG_HOOKS_FORMAT_ATTRIBUTE_TABLE): Define.

gcc/testsuite/ChangeLog:
	PR jit/95426
	* jit.dg/all-non-failing-tests.h: Add note about...
	* jit.dg/test-builtin-unreachable.c: New test.