bsd-user: Define TARGET_SIGSTACK_ALIGN and use it to round stack

Most (all?) targets require stacks to be properly aligned. Rather than a
series of ifdefs in bsd-user/signal.h, instead use a manditory #define
for all architectures.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

bsd-user/aarch64/target_arch_signal.h

@@ -77,4 +77,6 @@ struct target_sigframe {
     target_ucontext_t   sf_uc;  /* saved ucontext */
 };
 
+#define TARGET_SIGSTACK_ALIGN 16
+
 #endif /* TARGET_ARCH_SIGNAL_H */

bsd-user/arm/target_arch_signal.h

@@ -86,4 +86,6 @@ struct target_sigframe {
     target_mcontext_vfp_t sf_vfp; /* actual saved VFP context */
 };
 
+#define TARGET_SIGSTACK_ALIGN 8
+
 #endif /* TARGET_ARCH_SIGNAL_H */

bsd-user/i386/target_arch_signal.h

@@ -88,4 +88,6 @@ struct target_sigframe {
     uint32_t    __spare__[2];
 };
 
+#define TARGET_SIGSTACK_ALIGN 8
+
 #endif /* TARGET_ARCH_SIGNAL_H */

bsd-user/signal.c

@@ -728,14 +728,7 @@ static inline abi_ulong get_sigframe(struct target_sigaction *ka,
         sp = ts->sigaltstack_used.ss_sp + ts->sigaltstack_used.ss_size;
     }
 
-/* TODO: make this a target_arch function / define */
-#if defined(TARGET_ARM)
-    return (sp - frame_size) & ~7;
-#elif defined(TARGET_AARCH64)
-    return (sp - frame_size) & ~15;
-#else
-    return sp - frame_size;
-#endif
+    return ROUND_DOWN(sp - frame_size, TARGET_SIGSTACK_ALIGN);
 }
 
 /* compare to $M/$M/exec_machdep.c sendsig and sys/kern/kern_sig.c sigexit */

bsd-user/x86_64/target_arch_signal.h

@@ -97,4 +97,6 @@ struct target_sigframe {
     uint32_t    __spare__[2];
 };
 
+#define TARGET_SIGSTACK_ALIGN 16
+
 #endif /* TARGET_ARCH_SIGNAL_H */