Suppress incorrect objtool warnings

Suppress incorrect warnings from versions of objtool which are not
aware of x86 EVEX prefix instructions used for AVX512.

  module/zfs/vdev_raidz_math_avx512bw.o: warning:
  objtool: <func+offset>: can't find jump dest instruction at .text

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6928