CMakeLists: Remove -march=native options in case of cross compilation

This option can break cross-compilations of the program

In my test yocto CFLAGS -march and -mtune were ignored due to
the -march=native.

root@qt5022:~# clpeak
Platform: AMD Accelerated Parallel Processing
  Device: Loveland
    Driver version  : 1598.5 (Linux x64)
    Compute units   : 2
    Clock frequency : 507 MHz
Illegal instruction

[  822.855887] traps: clpeak[1311] trap invalid opcode ip:405045
	sp:7fffd2429260 error:0 in clpeak[400000+20000]

1  0x000000000040cbba in clPeak::runGlobalBandwidthTest(cl::CommandQueue&,
2  0x0000000000406992 in clPeak::runAll() ()
3  0x0000000000404bfe in main ()
src/common: Implement roundtoPowOf2 with binary logic

Instead of using expensive log instructions with floats.