bsd-user/arm/target_arch_signal.h: Define size of *context_t

Define the native sizes of mcontext_t and ucontext_t so that the tests
in target_os_ucontext.h ensure the size of arm's version of these
structures is correct.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/bsd-user/arm/target_arch_signal.h b/bsd-user/arm/target_arch_signal.h
index 9527335..f1844db 100644
--- a/bsd-user/arm/target_arch_signal.h
+++ b/bsd-user/arm/target_arch_signal.h
@@ -74,6 +74,9 @@
     abi_int     mc_spare[33];
 } target_mcontext_t;
 
+#define TARGET_MCONTEXT_SIZE 208
+#define TARGET_UCONTEXT_SIZE 260
+
 #include "target_os_ucontext.h"
 
 struct target_sigframe {