objtool: Enclose contents of unreachable() macro in a block

Guenter Roeck reported a boot failure in mips64.  It was bisected to the
following commit:

  d1091c7f ("objtool: Improve detection of BUG() and other dead ends")

The unreachable() macro was formerly only composed of a single
statement.  The above commit added a second statement, but neglected to
enclose the statements in a block.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: d1091c7f ("objtool: Improve detection of BUG() and other dead ends")
Link: http://lkml.kernel.org/r/20170228042116.glmwmwiohcix7o4a@treble


Signed-off-by: Ingo Molnar <mingo@kernel.org>