libc: Add ftruncate64 and improve 64-bit parameter syscall handling.

This patch improves the handling of 64-bit parameters in syscalls on ARM.
The ARM EABI mandates that 64-bit quantities be passed in even/odd register
pairs, which requires special treatment.

This allows us to simplify our implementations of pread() and pwrite()
and remove the C stubs for pread64() and pwrite64().

Also add ftruncate64() to <unistd.h>

Change-Id: I407e2fd223ba0093dd2d0b04c6152fadfc9ce3ef

Bug 3107933
diff --git a/libc/arch-x86/syscalls/pwrite64.S b/libc/arch-x86/syscalls/pwrite64.S
new file mode 100644
index 0000000..01389f8
--- /dev/null
+++ b/libc/arch-x86/syscalls/pwrite64.S
@@ -0,0 +1,35 @@
+/* autogenerated by gensyscalls.py */
+#include <sys/linux-syscalls.h>
+
+    .text
+    .type pwrite64, @function
+    .globl pwrite64
+    .align 4
+
+pwrite64:
+    pushl   %ebx
+    pushl   %ecx
+    pushl   %edx
+    pushl   %esi
+    pushl   %edi
+    mov     24(%esp), %ebx
+    mov     28(%esp), %ecx
+    mov     32(%esp), %edx
+    mov     36(%esp), %esi
+    mov     40(%esp), %edi
+    movl    $__NR_pwrite64, %eax
+    int     $0x80
+    cmpl    $-129, %eax
+    jb      1f
+    negl    %eax
+    pushl   %eax
+    call    __set_errno
+    addl    $4, %esp
+    orl     $-1, %eax
+1:
+    popl    %edi
+    popl    %esi
+    popl    %edx
+    popl    %ecx
+    popl    %ebx
+    ret