Build fails on ARM64 with Clang

The build currently fails on ARM64 with Clang.

cc -Itests/59830eb@@checkasm_bitdepth_8@sta -Itests -I../dav1d-0.2.1/tests -I. -I../dav1d-0.2.1/ -Iinclude/dav1d -I../dav1d-0.2.1/include/dav1d -Iinclude -I../dav1d-0.2.1/include -Xclang -fcolor-diagnostics -pipe -D_FILE_OFFSET_BITS=64 -std=c99 -D_POSIX_C_SOURCE=200112L -fvisibility=hidden -Wundef -Werror=vla -Wno-missing-field-initializers -Wno-unused-parameter -Werror=missing-prototypes -Wshorten-64-to-32 -O2 -fPIC -DBITDEPTH=8 -MD -MQ 'tests/59830eb@@checkasm_bitdepth_8@sta/checkasm_ipred.c.o' -MF 'tests/59830eb@@checkasm_bitdepth_8@sta/checkasm_ipred.c.o.d' -o 'tests/59830eb@@checkasm_bitdepth_8@sta/checkasm_ipred.c.o' -c ../dav1d-0.2.1/tests/checkasm/ipred.c
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:152:33: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
                         : "=r"(cycle_counter)
                                ^
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:44: note: use constraint modifier "w"
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                                           ^~
                                           %w0
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x23, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x8, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x20, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x8, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x24, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x8, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x20, c9, c13, 0
^
In file included from ../dav1d-0.2.1/tests/checkasm/ipred.c:28:
../dav1d-0.2.1/tests/checkasm/checkasm.h:151:32: error: unrecognized instruction mnemonic, did you mean: mrs, msr, smc?
    __asm__ __volatile__("isb\nmrc p15, 0, %0, c9, c13, 0"
                               ^
<inline asm>:2:1: note: instantiated into assembly here
mrc p15, 0, x8, c9, c13, 0
^
1 warning and 8 errors generated.
ninja: build stopped: subcommand failed.

Full build log

http://build-failures.rhaalovely.net/aarch64/2019-03-27/multimedia/dav1d.log