x86/mce: Make machine check speculation protected

The machine check idtentry uses an indirect branch directly from the low
level code. This evades the speculation protection.

Replace it by a direct call into C code and issue the indirect call there
so the compiler can apply the proper speculation protection.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Niced-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801181626290.1847@nanos