Do not try to do runtime tests during cross compile
Fixes
| CMake Error at Source/cmake/OptionsCommon.cmake:73 (math):
| math cannot parse the expression: "-1": syntax error, unexpected exp_MINUS,
| expecting exp_OPENPARENT or exp_NUMBER (1)
(From OE-Core rev: 528006009dddd876a830e0a8f248658182a37f37)
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>