exec: Move [b]tswapl() declarations to 'exec/user/tswap-target.h'

tswapl() and bswaptls() are target-dependent and only used
by user emulation. Move their definitions to a new header:
"exec/user/tswap-target.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20231212123401.37493-17-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/i386/signal.c

@@ -21,6 +21,7 @@
 #include "user-internals.h"
 #include "signal-common.h"
 #include "linux-user/trace.h"
+#include "user/tswap-target.h"
 
 /* from the Linux kernel - /arch/x86/include/uapi/asm/sigcontext.h */