Have coproc under #ifdef ARM

[andr2000]

Currently coprocs are for ARM only, but generic code, like libxl, has code changes w/o the corresponding ifdef

[andr2000]

Alternative way is to add stubs for x86

[aanisov]

Theoretically we are targeting all arms and x86. But have in short term plans only arm implementation yet. So x86 stubs looks like more generic approach.