Workaround build failure on ia64

Avoid 'o' constraint to allow gcc to select register operand.
That way instruction selector can satisfy all constraints
for PROBE2 macro.

Reported-by: Émeric Maschino
Bug: https://bugs.gentoo.org/510134
--- a/cache.cxx
+++ b/cache.cxx
@@ -8,2 +8,10 @@
 
+#ifdef __ia64__
+// Default is: nor
+// We disable 'o' because gcc fails to select register
+// constraint for second instruction in 'add_script_to_cache'
+//     https://bugs.gentoo.org/510134
+#    define STAP_SDT_ARG_CONSTRAINT nr
+#endif
+
 #include "config.h"
--- a/staprun/mainloop.c
+++ b/staprun/mainloop.c
@@ -1 +1,9 @@
+#ifdef __ia64__
+// Default is: nor
+// We disable 'o' because gcc fails to select register
+// constraint for second instruction in 'add_script_to_cache'
+//     https://bugs.gentoo.org/510134
+#    define STAP_SDT_ARG_CONSTRAINT nr
+#endif
+
 /* -*- linux-c -*-