Turn all use sites of __builtin_return_address(0) to use the newly
defined rtld_get_return_address macro, which is overridden to take into
account the existence of trampolines when c18n is enabled.

In the following example, bar makes a tail-call to foo, which returns a
value that is observable to the caller of bar, even though bar returns
nothing.

    void *foo();
    void bar() { foo(); }

When bar is called, previous versions of the trampoline clears return
value registers as if foo is being called directly, leaking a
capability. Instead, clear the maximum number of return value registers
as required for both foo and bar.