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-arm/syscalls/ftruncate64.S b/libc/arch-arm/syscalls/ftruncate64.S
new file mode 100644
index 0000000..37b4744
--- /dev/null
+++ b/libc/arch-arm/syscalls/ftruncate64.S
@@ -0,0 +1,19 @@
+/* autogenerated by gensyscalls.py */
+#include <sys/linux-syscalls.h>
+
+    .text
+    .type ftruncate64, #function
+    .globl ftruncate64
+    .align 4
+    .fnstart
+
+ftruncate64:
+    .save   {r4, r7}
+    stmfd   sp!, {r4, r7}
+    ldr     r7, =__NR_ftruncate64
+    swi     #0
+    ldmfd   sp!, {r4, r7}
+    movs    r0, r0
+    bxpl    lr
+    b       __set_syscall_errno
+    .fnend