[nvptx] Fix boolean type test in write_fn_proto

[nvptx] Fix boolean type test in write_fn_proto

When running this libgomp testcase for nvptx accelerator:
...
/* { dg-do run } */
__uint128_t v;
int main () {
  #pragma omp target
  {
    __uint128_t exp = 2;
    __atomic_compare_exchange_n (&v, &exp, 7, false, __ATOMIC_RELEASE,
				 __ATOMIC_ACQUIRE);
  }
}
...
we run into this assert in write_fn_proto:
...
913             gcc_assert (type == boolean_type_node);
...

This happens when doing some special-handling code for
__atomic_compare_exchange_1/2/4/8/16.  The function decls have a parameter
called weak of type bool, which is skipped when writing the decl because
the corresponding libatomic functions do not have that parameter.  The assert
is there to verify that we skip the correct parameter.

However, we assert because we have different type of bools:
...
(gdb) call debug_generic_expr (type)
_Bool
(gdb) call debug_generic_expr (global_trees[TI_BOOLEAN_TYPE])
bool
...

Fix this by checking for TREE_CODE (type) == BOOLEAN_TYPE instead.

Tested libgomp on x86_64-linux with nvptx accelerator.

Likewise, tested that the test-case above does not ICE anymore.

gcc/ChangeLog:

	PR target/96991
	* config/nvptx/nvptx.c (write_fn_proto): Fix boolean type check.