Prioritize the constructor call of __local_xray_dyninit()

[krytarowski]

Summary

For platforms without preinit support (such as NetBSD/amd64) the
initialization routine __xray_init() was called in non-deterministic order
compared to other constructors. This caused breakage failures
as xray routines attempted to execute code with assumption of
being initialized, which was no always true.

Use GCC/Clang extension to set maximal priority to the constructor
calling __xray_init(). This code switches away from C++ lambda form,
as it did not allow to specify this compiler extension.

[dberris]

LGTM

Has this been tested with GCC/Clang on other systems (like Linux)?

[mgorny]

This isn't normally used on Linux since .preinit_array is used. If I force it, I get the following warning (from gcc):

../projects/compiler-rt/lib/xray/xray_init.cc:110:34: warning: constructor priorities from 0 to 100 are reserved for the implementation
 static void __local_xray_dyninit() {
                                  ^

All tests pass.

lib/xray/xray_init.cc

 #if !defined(XRAY_NO_PREINIT) &&                                               \
     (SANITIZER_CAN_USE_PREINIT_ARRAY || SANITIZER_FREEBSD)
 // Only add the preinit array initialization if the sanitizers can.
 __attribute__((section(".preinit_array"),
                used)) void (*__local_xray_preinit)(void) = __xray_init;
 #else
 // If we cannot use the .preinit_array section, we should instead use dynamic
 // initialisation.
-static bool UNUSED __local_xray_dyninit = [] {
+__attribute__ ((constructor (0)))
+static void __local_xray_dyninit() {
   __xray_init();
-  return true;
-}();
+}
 #endif