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

Change-Id: Ibcd1e53b03f3c8db91a3cd8825d06bcf8f43f443
Reviewed-on: https://go-review.googlesource.com/50635
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/syscall/libcall_linux.go b/libgo/go/syscall/libcall_linux.go
index b58b2dd..5f47784 100644
--- a/libgo/go/syscall/libcall_linux.go
+++ b/libgo/go/syscall/libcall_linux.go
@@ -212,7 +212,7 @@
 //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 @@
 }
 
 //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