libstdc++: Simplify <ext/aligned_buffer.h> class templates

As noted in a comment, the __gnu_cxx::__aligned_membuf class template
can be simplified, because alignof(T) and alignas(T) use the correct
alignment for a data member. That's true since GCC 8 and Clang 8. The
EDG front end (as used by Intel icc, aka "Intel C++ Compiler Classic")
does not implement the PR c++/69560 change, so keep using the old
implementation when __EDG__ is defined, to avoid an ABI change for icc.

For __gnu_cxx::__aligned_buffer<T> all supported compilers agree on the
value of __alignof__(T), but we can still simplify it by removing the
dependency on std::aligned_storage<sizeof(T), __alignof__(T)>.

Add a test that checks that the aligned buffer types have the expected
alignment, so that we can tell if changes like this affect their ABI
properties.

libstdc++-v3/ChangeLog:

	* include/ext/aligned_buffer.h (__aligned_membuf): Use
	alignas(T) directly instead of defining a struct and using 9its
	alignment.
	(__aligned_buffer): Remove use of std::aligned_storage.
	* testsuite/abi/aligned_buffers.cc: New test.

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