[libc] Add x86_64 versions of setjmp() and longjmp()

None of the x86_64 builds currently have any way of invoking these
functions.  They are included only to avoid introducing unnecessary
architecture-specific dependencies into the self-test suite.

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

src/arch/x86_64/include/setjmp.h

@@ -0,0 +1,34 @@
+#ifndef _SETJMP_H
+#define _SETJMP_H
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdint.h>
+
+/** A jump buffer */
+typedef struct {
+	/** Saved return address */
+	uint64_t retaddr;
+	/** Saved stack pointer */
+	uint64_t stack;
+	/** Saved %rbx */
+	uint64_t rbx;
+	/** Saved %rbp */
+	uint64_t rbp;
+	/** Saved %r12 */
+	uint64_t r12;
+	/** Saved %r13 */
+	uint64_t r13;
+	/** Saved %r14 */
+	uint64_t r14;
+	/** Saved %r15 */
+	uint64_t r15;
+} 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 */