[libc] Centralise architecture-independent portions of setjmp.h

The definitions of the setjmp() and longjmp() functions are common to
all architectures, with only the definition of the jump buffer
structure being architecture-specific.

Move the architecture-specific portions to bits/setjmp.h and provide a
common setjmp.h for the function definitions.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/setjmp.h

@@ -0,0 +1,14 @@
+#ifndef _SETJMP_H
+#define _SETJMP_H
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <bits/setjmp.h>
+
+extern int __asmcall __attribute__ (( returns_twice ))
+setjmp ( jmp_buf env );
+
+extern void __asmcall __attribute__ (( noreturn ))
+longjmp ( jmp_buf env, int val );
+
+#endif /* _SETJMP_H */