cmake/modules/BuildISAL.cmake: use GNU assembler

cmake/modules/BuildISAL.cmake: use GNU assembler

avoid the following error by disabling clang assembler:
```
[670/2691] Performing build step for 'isal_ext'
FAILED: src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build src/isa-l/install/lib/libisal.a /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/isa-l/install/lib/libisal.a
cd /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/isa-l && /usr/bin/cmake -P /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build-Debug.cmake && /usr/bin/cmake -E touch /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build
CMake Error at /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build-Debug.cmake:37 (message):
  Command failed: 2

   '/usr/bin/cmake' '-E' 'env' '--unset=DESTDIR' 'make' '-j3'

  See also

    /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-build.log

-- Log output is:
...skipping to end...
hile in macro instantiation
crc64_refl_func crc64_ecma_refl_pmull
^
<instantiation>:2:26: error: unexpected token in argument list
 movk x7, p1_low_b1, lsl 16
                         ^
<instantiation>:54:2: note: while in macro instantiation
 crc64_fold_512b_to_128b
 ^
crc/aarch64/crc64_ecma_refl_pmull.S:33:1: note: while in macro instantiation
crc64_refl_func crc64_ecma_refl_pmull
^
<instantiation>:3:26: error: unexpected token in argument list
 movk x7, p1_low_b2, lsl 32
                         ^
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>