x86/entry: __always_inline debugreg for noinstr

vmlinux.o: warning: objtool: exc_debug()+0x21: call to native_get_debugreg() leaves .noinstr.text section

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200603114051.954401211@infradead.org