```
coroutine/arm64/Context.S:31:57: error: invoking macro TOKEN_PASTE argument 1: empty macro arguments are undefined in ISO C90 [-Wpedantic]
   31 | .global PREFIXED_SYMBOL(SYMBOL_PREFIX,coroutine_transfer)
      |                                                         ^
```

These apparently break compilation on old MacOS toolchains, because the
MachO section name is capped to 16 chars (although, on my MacOS, at
least, the section name just gets truncated). Nevertheless, these serve
no purpose on non-ELF platforms (they're part of the LSB Linux ABI).

[Bug #20677]

We do not implement CET shadow-stack switching in amd64 Context.S. If
you compile Ruby with `-fcf-protection=full` and run it with
`GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK` exported, it will crash with a
control flow exception.

Configure the appropriate notes at the end of Context.S

[Bug #18061]

According to the GCC documentation [1], the macro `__CET__` is dfined
when `-fcf-protection` compiler option is used. Therefore use this macro
to enable IBT support instead of special casing for `__OpenBSD__`.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fcf-protection

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.

Not enabling for ELF in general as not all platform support it
 (e.g. NetBSD, implictly stack never executable).