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

libc/arch-arm/syscalls/pwrite64.S

@@ -0,0 +1,21 @@
+/* autogenerated by gensyscalls.py */
+#include <sys/linux-syscalls.h>
+
+    .text
+    .type pwrite64, #function
+    .globl pwrite64
+    .align 4
+    .fnstart
+
+pwrite64:
+    mov     ip, sp
+    .save   {r4, r5, r6, r7}
+    stmfd   sp!, {r4, r5, r6, r7}
+    ldmfd   ip, {r4, r5, r6}
+    ldr     r7, =__NR_pwrite64
+    swi     #0
+    ldmfd   sp!, {r4, r5, r6, r7}
+    movs    r0, r0
+    bxpl    lr
+    b       __set_syscall_errno
+    .fnend