add powerpc64 port

src/internal/powerpc64/syscall.s

@@ -0,0 +1,17 @@
+	.global __syscall
+	.hidden __syscall
+	.type   __syscall,@function
+__syscall:
+	mr      0, 3                  # Save the system call number
+	mr      3, 4                  # Shift the arguments: arg1
+	mr      4, 5                  # arg2
+	mr      5, 6                  # arg3
+	mr      6, 7                  # arg4
+	mr      7, 8                  # arg5
+	mr      8, 9                  # arg6
+	sc
+	bnslr+       # return if not summary overflow
+	neg     3, 3 # otherwise error: return negated value.
+	blr
+	.end    __syscall
+	.size   __syscall, .-__syscall