bsd-user: create a per-arch signal.c file

Create a place-holder signal.c file for each of the architectures that
are currently built. In the future, some code that's currently inlined
in target_arch_signal.h will live here.

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

bsd-user/i386/signal.c

@@ -0,0 +1 @@
+/* Placeholder for signal.c */

bsd-user/x86_64/signal.c

@@ -0,0 +1 @@
+/* Placeholder for signal.c */

meson.build

@@ -2933,7 +2933,7 @@ foreach target : target_dirs
       base_dir = 'bsd-user'
       target_inc += include_directories('bsd-user/' / targetos)
       dir = base_dir / abi
-      arch_srcs += files(dir / 'target_arch_cpu.c')
+      arch_srcs += files(dir / 'signal.c', dir / 'target_arch_cpu.c')
     endif
     target_inc += include_directories(
       base_dir,