cmake/modules/BuildISAL.cmake: set no-integrated-as on clang only

cmake/modules/BuildISAL.cmake: set no-integrated-as on clang only

this option is only relevant to clang, gcc will fail with:
```
CMake Error at ceph/build/src/erasure-code/isa/isal_ext-prefix/src/isal_ext-stamp/isal_ext-configure-Debug-impl.cmake:19 (message):
  Command failed (77):

   'env' 'CC=/usr/bin/gcc-11' './configure' '--prefix=ceph/build/src/isa-l/install' '--with-pic' '--enable-static' '--disable-shared' 'CFLAGS=-no-integrated-as'
```

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