gdbsupport: assume that compiler supports std::{is_trivially_constructible,is_trivial...

gdbsupport: assume that compiler supports std::{is_trivially_constructible,is_trivially_copyable}

This code was there to support g++ 4, which didn't support
std::is_trivially_constructible and std::is_trivially_copyable.  Since
we now require g++ >= 9, I think it's fair to assume that GDB will
always be compiled with a compiler that supports those.

Change-Id: Ie7c1649139a2f48bf662cac92d7f3e38fb1f1ba1