Sync up with NetBSD libexecinfo changes 2014-2019

Sync up with NetBSD libexecinfo changes 2014-2019

Drop portions that are unlit or redundant with llvm-libunwind: builtin.c,
unwind.h, and unwind_arm_ehabi_stub.c.

This code should now work with -fPIE binaries, should we choose to build any
that way.

When backtrace() array is full, signal an error so the underlying
Itanium-style C++ exception handling library (llvm-libunwind) knows to stop
tracing instead of continuing.  (It should stop on its own when it finishes
unwinding, so this is mostly an extra seatbelt against an infinite loop bug
in the unwinder.)