fix pread/pwrite syscall calling convention on sh

despite sh not generally using register-pair alignment for 64-bit
syscall arguments, there are arch-specific versions of the syscall
entry points for pread and pwrite which include a dummy argument for
alignment before the 64-bit offset argument.

arch/sh/syscall_arch.h

@@ -2,6 +2,7 @@
 ((union { long long ll; long l[2]; }){ .ll = x }).l[0], \
 ((union { long long ll; long l[2]; }){ .ll = x }).l[1]
 #define __SYSCALL_LL_O(x) __SYSCALL_LL_E((x))
+#define __SYSCALL_LL_PRW(x) 0, __SYSCALL_LL_E((x))
 
 /* The extra OR instructions are to work around a hardware bug:
  * http://documentation.renesas.com/doc/products/mpumcu/tu/tnsh7456ae.pdf