syscall: remove calls to Entersyscall & Exitsyscall in Getdents

The syscall for Getdents in syscall/libcall_linux.go
    called Entersyscall and Exitsyscall, causing the
    runtime_sched counts for goroutines to be incorrect.
    Inconsistent counts caused checkdead in runtime/proc.c
    to panic.
    
    Fixes golang/go#11406
    
    Reviewed-on: https://go-review.googlesource.com/11761

From-SVN: r225745

libgo/go/syscall/libcall_linux.go

@@ -223,7 +223,6 @@ func Getdents(fd int, buf []byte) (n int, err error) {
 	} else {
 		p = (*byte)(unsafe.Pointer(&_zero))
 	}
-	Entersyscall()
 	s := SYS_GETDENTS64
 	if s == 0 {
 		s = SYS_GETDENTS
@@ -233,7 +232,6 @@ func Getdents(fd int, buf []byte) (n int, err error) {
 	if n < 0 {
 		err = errno
 	}
-	Exitsyscall()
 	return
 }