UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36

Steps to reproduce the problem:
1. Apply CL https://chromium-review.googlesource.com/c/567941/
2. make BOARD=elm board/elm/board.o

What is the expected behavior?
The step 2 only reports warnings, no errors

What went wrong?
Clang doesn't support the operand constraint "b".

The following is the error message:

In file included from board/elm/board.c:10:
core/cortex-m0/atomic.h:32:2: error: invalid output constraint '=&b' in asm
        ATOMIC_OP(bic, addr, bits);
        ^
core/cortex-m0/atomic.h:26:11: note: expanded from macro 'ATOMIC_OP'
                             : "=&b" (reg0)                     \
                               ^
core/cortex-m0/atomic.h:37:2: error: invalid output constraint '=&b' in asm
        ATOMIC_OP(orr, addr, bits);
        ^
core/cortex-m0/atomic.h:26:11: note: expanded from macro 'ATOMIC_OP'
                             : "=&b" (reg0)                     \
                               ^
core/cortex-m0/atomic.h:42:2: error: invalid output constraint '=&b' in asm
        ATOMIC_OP(add, addr, value);
        ^
core/cortex-m0/atomic.h:26:11: note: expanded from macro 'ATOMIC_OP'
                             : "=&b" (reg0)                     \
                               ^
core/cortex-m0/atomic.h:47:2: error: invalid output constraint '=&b' in asm
        ATOMIC_OP(sub, addr, value);
        ^
core/cortex-m0/atomic.h:26:11: note: expanded from macro 'ATOMIC_OP'
                             : "=&b" (reg0)                     \
                               ^
core/cortex-m0/atomic.h:59:11: error: invalid output constraint '=&b' in asm
                             : "=&b" (ret)
                               ^

Did this work before? N/A 

Chrome version: 59.0.3071.115  Channel: stable
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