DJGPP (and maybe other targets) uses MAX_OFILE_ALIGNMENT=16 which means
that globals (and static objects) can't have alignment greater than 16.
This causes an error for the locks defined in src/c++11/shared_ptr.cc
because we try to align them to the cacheline size, to avoid false
sharing.

Add a configure check for the increased alignment, and live with false
sharing where we can't increase the alignment.

libstdc++-v3/ChangeLog:

	PR libstdc++/109741
	* acinclude.m4 (GLIBCXX_CHECK_ALIGNAS_CACHELINE): Define.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_CHECK_ALIGNAS_CACHELINE.
	* src/c++11/shared_ptr.cc (__gnu_internal::get_mutex): Do not
	align lock table if not supported. use __GCC_DESTRUCTIVE_SIZE
	instead of hardcoded 64.