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

gcc/go/gofrontend/MERGE

-b712bacd939466e66972337744983e180849c535
+e1bd9ea4dc16e228164c92a12c5229ddf20f2b50
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/libcall_linux.go

@@ -212,7 +212,7 @@ func Accept4(fd int, flags int) (nfd int, sa Sockaddr, err error) {
 //flock(fd _C_int, how _C_int) _C_int
 
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
-//fstatfs(fd _C_int, buf *Statfs_t) _C_int
+//fstatfs64(fd _C_int, buf *Statfs_t) _C_int
 
 func Gettid() (tid int) {
 	r1, _, _ := Syscall(SYS_GETTID, 0, 0, 0)
@@ -360,7 +360,7 @@ func Splice(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n i
 }
 
 //sys	Statfs(path string, buf *Statfs_t) (err error)
-//statfs(path *byte, buf *Statfs_t) _C_int
+//statfs64(path *byte, buf *Statfs_t) _C_int
 
 //sys	SyncFileRange(fd int, off int64, n int64, flags int) (err error)
 //sync_file_range(fd _C_int, off Offset_t, n Offset_t, flags _C_uint) _C_int