syscall: fix mkall.sh, mksyscall_linux.pl, and regen for Linux/ARM

CL 3075041 says ARM is not little-endian, but my test suggests otherwise.
My test program is:

    package main
    import ("fmt"; "syscall"; "os")
    func main() {
       err := syscall.Fallocate(1, 1/*FALLOC_FL_KEEP_SIZE*/, 0, int64(40960));
       fmt.Fprintln(os.Stderr, err)
    }

Without this CL, ./test > testfile will show: file too large; and strace shows:
    fallocate(1, 01, 0, 175921860444160)    = -1 EFBIG (File too large)
With this CL, ./test > testfile will show: <nil>; and strace shows:
    fallocate(1, 01, 0, 40960)              = 0

Quoting rsc:
"[It turns out that] ARM syscall ABI requires 64-bit arguments to use an
(even, odd) register pair, not an (odd, even) pair. Switching to "big-endian"
worked because it ended up using the high 32-bits (always zero in the tests
we had) as the padding word, because the 64-bit argument was the last one,
and because we fill in zeros for the rest of the system call arguments, up to
six. So it happened to work."

I updated mksyscall_linux.pl to accommodate the register pair ABI requirement,
and removed all hand-tweaked syscall routines in favor of the auto-generated
ones. These including: Ftruncate, Truncate, Pread and Pwrite.

Some recent Linux/ARM distributions do not bundle kernel asm headers,
so instead we always get latest asm/unistd.h from git.kernel.org (just like
what we do for FreeBSD).

R=ken, r, rsc, r, dave, iant
CC=golang-dev
https://golang.org/cl/5726051

src/pkg/syscall/zsyscall_linux_arm.go

@@ -1,4 +1,4 @@
-// mksyscall.pl -b32 syscall_linux.go syscall_linux_arm.go
+// mksyscall.pl -l32 -arm syscall_linux.go syscall_linux_arm.go
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
 package syscall
@@ -287,7 +287,7 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
 func Fallocate(fd int, mode uint32, off int64, len int64) (err error) {
-	_, _, e1 := Syscall6(SYS_FALLOCATE, uintptr(fd), uintptr(mode), uintptr(off>>32), uintptr(off), uintptr(len>>32), uintptr(len))
+	_, _, e1 := Syscall6(SYS_FALLOCATE, uintptr(fd), uintptr(mode), uintptr(off), uintptr(off>>32), uintptr(len), uintptr(len>>32))
 	if e1 != 0 {
 		err = e1
 	}
@@ -784,7 +784,7 @@ func Sysinfo(info *Sysinfo_t) (err error) {
 
 func Tee(rfd int, wfd int, len int, flags int) (n int64, err error) {
 	r0, r1, e1 := Syscall6(SYS_TEE, uintptr(rfd), uintptr(wfd), uintptr(len), uintptr(flags), 0, 0)
-	n = int64(int64(r0)<<32 | int64(r1))
+	n = int64(int64(r1)<<32 | int64(r0))
 	if e1 != 0 {
 		err = e1
 	}
@@ -1458,6 +1458,60 @@ func Time(t *Time_t) (tt Time_t, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func Pread(fd int, p []byte, offset int64) (n int, err error) {
+	var _p0 unsafe.Pointer
+	if len(p) > 0 {
+		_p0 = unsafe.Pointer(&p[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero)
+	}
+	r0, _, e1 := Syscall6(SYS_PREAD64, uintptr(fd), uintptr(_p0), uintptr(len(p)), 0, uintptr(offset), uintptr(offset>>32))
+	n = int(r0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func Pwrite(fd int, p []byte, offset int64) (n int, err error) {
+	var _p0 unsafe.Pointer
+	if len(p) > 0 {
+		_p0 = unsafe.Pointer(&p[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero)
+	}
+	r0, _, e1 := Syscall6(SYS_PWRITE64, uintptr(fd), uintptr(_p0), uintptr(len(p)), 0, uintptr(offset), uintptr(offset>>32))
+	n = int(r0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func Truncate(path string, length int64) (err error) {
+	_, _, e1 := Syscall6(SYS_TRUNCATE64, uintptr(unsafe.Pointer(StringBytePtr(path))), 0, uintptr(length), uintptr(length>>32), 0, 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func Ftruncate(fd int, length int64) (err error) {
+	_, _, e1 := Syscall6(SYS_FTRUNCATE64, uintptr(fd), 0, uintptr(length), uintptr(length>>32), 0, 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func mmap2(addr uintptr, length uintptr, prot int, flags int, fd int, pageOffset uintptr) (xaddr uintptr, err error) {
 	r0, _, e1 := Syscall6(SYS_MMAP2, uintptr(addr), uintptr(length), uintptr(prot), uintptr(flags), uintptr(fd), uintptr(pageOffset))
 	xaddr = uintptr(r0)