db: disable the ARM assembler mutex code

db: disable the ARM assembler mutex code

The swpb in macro MUTEX_SET will cause "undefined instruction" error
on the new arm arches which don't support this assembly instruction
any more. If use ldrex/strex to replace swpb, the old arm arches don't
support them. So to avoid this issue, just disable the ARM assembler
mutex code, and use the default pthreads mutex.

(From OE-Core rev: aafbc548ebc66dc0d703526f9a98f784e9c9605b)

Signed-off-by: Li Zhou <li.zhou@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>