libstdc++: Revert change to __bitwise_relocatable

This reverts r15-6060-ge4a0157c2397c9 so that __is_bitwise_relocatable
depends only on is_trivial. To avoid the deprecation warnings for C++26,
use the __is_trivial built-in directly instead of std::is_trivial.

We need to be sure that the type is trivially copyable, not just
trivially constructible and trivially assignable. Otherwise we get
-Wclass-memaccess diagnostics for e.g. std::vector<std::pair<A*, B*>>.
We could add is_trivially_copyable to the conditions, but this isn't
really an appropriate change for stage 3 anyway (it affects all modes
from C++11 upwards). Just revert to using is_trivial, and we can revisit
the condition for GCC 16.

libstdc++-v3/ChangeLog:

	* include/bits/stl_uninitialized.h (__is_bitwise_relocatable):
	Revert to depending on is_trivial.