[llvm-exegesis][AArch64] Adding PR_PAC_APIAKEY macro definition (llvm#137888)

In older Linux distros, PR_PAC_APIAKEY is not defined and it causes
build failures on linux arm64 platform. This patch adds the definition
of this macro if it is not defined.

Author: zeroomega

llvm/tools/llvm-exegesis/lib/AArch64/Target.cpp

@@ -18,6 +18,9 @@
 #ifndef PR_PAC_GET_ENABLED_KEYS
 #define PR_PAC_GET_ENABLED_KEYS 61
 #endif
+#ifndef PR_PAC_APIAKEY
+#define PR_PAC_APIAKEY (1UL << 0)
+#endif
 #endif
 
 #define GET_AVAILABLE_OPCODE_CHECKER