syscall: call f?statfs64 on GNU/Linux

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

From-SVN: r250443