nptl: compile pt-vfork in ARM-mode

amending commit 49a67cf67d5a7194214566bc730ee7e28d55bbe1
could need a thumb implementation..

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libpthread/nptl/sysdeps/unix/sysv/linux/arm/Makefile.arch

@@ -18,3 +18,6 @@ libc_linux_arch_SSRC-OMIT = waitpid.S
 # This macro should be alternatively implemented in THUMB
 # assembly.
 ASFLAGS-pt-vfork.S = -marm
+CFLAGS-OMIT-pt-vfork.S = -mthumb
+ASFLAGS-vfork.S = -marm
+CFLAGS-OMIT-vfork.S = -mthumb