spl/thread: explicitly define thread_func_t as noreturn

All of our thread entry functions have this signature:

    void (*)(void*) __attribute__((noreturn))

The low-level __thread_create() function accepts a thread_func_t as
the entry point, which is defined more simply as:

    void (*)(void *)

And then the thread_create() and thread_create_named() macros cast
the passed-in function point down to thread_func_t, that is, casting
away the noreturn attribute.

Clang considers casting between these two types to be invalid because
both the caller and the callee may have elided parts of the stack frame
save and restore, knowing that they won't be needed.

Recent Linux appears to be setting -Wcast-function-type-strict, which
causes this invalid cast to emit a warning, which with -Werror is
converted to an error, breaking the build.

This commit fixes this in the simplest possible way: adding noreturn
to the thread_func_t attribute. Since all our thread entry functions
already have this attribute, it's arguably a just a consistency fix
anyway.

I considered removing the casts in the macros, which silences the
warnings, but it turns out that Clang has a bug that won't emit this
error for implicit conversions, only explicit casts. So leaving them
there seems like a reasonable belt-and-suspenders approach. Also,
frankly, this whole mechanism seems a little undercooked inside LLVM, so
I'm content go with my intuition about the smallest, least invaisve
change.

NOTE: __thread_create is exported by spl.ko and has a
thread_func_t arg, so this is an ABI break. Whether that matters in
practice, I have no idea.

Further reading:

• https://github.com/llvm/llvm-project/commit/1aad641c793090b4d036c03e737df2ebe2c32c57
• https://github.com/llvm/llvm-project/issues/7325
• https://github.com/llvm/llvm-project/issues/41465

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16672
Closes #16673