Prioritize the constructor call of __local_xray_dyninit()

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.

Reviewers: dberris, joerg

Reviewed By: dberris

Subscribers: llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54136

llvm-svn: 346222