[scudo] Eliminate the runtime dependency on libc++abi

Making this variable non-static avoids the need for locking to ensure
that the initialization is thread-safe which in turns eliminates the
runtime dependency on libc++abi library (for __cxa_guard_acquire and
__cxa_guard_release) which makes it possible to link scudo against
pure C programs.

Differential Revision: https://reviews.llvm.org/D28757

llvm-svn: 292253