libstdc++: Simplify makefile rule for largefile-config.h (PR91947)

The previous rule could leave an incomplete file if the build was
interrupted, which would then not be remade if make was run again.

This makes the rule more robust by writing to a temporary file and only
moving it into place as the final step. It also simplifies the rule so
that only the essential macro definitions are written to the file, not
the explanatory comments and commented out #undef lines.

Also, the macro for enabling LFS on Mac OS X 10.5 is now set
unconditionally, which is a bug fix from upstream autoconf.

	PR libstdc++/91947
	* include/Makefile.am (${host_builddir}/largefile-config.h): Simplify
	rule.
	* include/Makefile.in: Regenerate.