libstdc++: Compile basic_file_stdio.cc for LFS

Instead of using fopen64, lseek64, and fstat64 we can just include
<bits/largefile-config.h> which defines _FILE_OFFSET_BITS=64 (and
similar target-specific macros). Then we can just use fopen, lseek and
fstat as normal, and they'll be the LFS versions if supported by the
target.

libstdc++-v3/ChangeLog:

	* config/io/basic_file_stdio.cc: Define LFS macros.
	(__basic_file<char>::open): Use fopen unconditionally.
	(get_file_offset): Use lseek unconditionally.
	(__basic_file<char>::seekoff): Likewise.
	(__basic_file<char>::showmanyc): Use fstat unconditionally.