When running on newer Intel processors supporting the feature,
OpenBSD enforces indirect branch tracking. Without this endbr64
instruction, jumps to the coroutine_transfer function result
in SIGILL on OpenBSD/amd64 when using such processors.
The OpenBSD Ruby ports have been using a patch similar to this
for the past two months.
From some research, cet.h has been supported by GCC for about
6 years and LLVM for about 4 years.
It is more conventional to use compiler to pre-process and
assemble the `.S` file rather than forcing Makefile to use `.s`.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
