[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/arch/i386/include/bits/setjmp.h

@@ -1,5 +1,5 @@
-#ifndef _SETJMP_H
-#define _SETJMP_H
+#ifndef _BITS_SETJMP_H
+#define _BITS_SETJMP_H
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
@@ -21,10 +21,4 @@ typedef struct {
 	uint32_t ebp;
 } jmp_buf[1];
 
-extern int __asmcall __attribute__ (( returns_twice ))
-setjmp ( jmp_buf env );
-
-extern void __asmcall __attribute__ (( noreturn ))
-longjmp ( jmp_buf env, int val );
-
-#endif /* _SETJMP_H */
+#endif /* _BITS_SETJMP_H */