libc: Add missing pipe2() declaration and implementation.

Change-Id: Iacb914bd6ac5adc60c5671e6fef680ede21f9b0c
diff --git a/libc/arch-arm/syscalls.mk b/libc/arch-arm/syscalls.mk
index c466166..2d944ca 100644
--- a/libc/arch-arm/syscalls.mk
+++ b/libc/arch-arm/syscalls.mk
@@ -66,6 +66,7 @@
 syscall_src += arch-arm/syscalls/fchmod.S
 syscall_src += arch-arm/syscalls/dup.S
 syscall_src += arch-arm/syscalls/pipe.S
+syscall_src += arch-arm/syscalls/pipe2.S
 syscall_src += arch-arm/syscalls/dup2.S
 syscall_src += arch-arm/syscalls/select.S
 syscall_src += arch-arm/syscalls/ftruncate.S