syscall: call f?statfs64 on GNU/Linux
We unconditionally set _FILE_OFFSET_BITS to 64 in configure.ac, so we
should unconditionally call the statfs64 and fstatfs64 functions.
These functions should be available on all versions of GNU/Linux since 2.6.
On 64-bit systems they are aliased to statfs/fstatfs, and on 32-bit
systems they use the 64-bit data structures.
Fixes golang/go#20922
Reviewed-on: https://go-review.googlesource.com/50635
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@250443 138bc75d-0d04-0410-961f-82ee72b054a4