patch devel/cpuflags default to use clang as CC when FreeBSD version >= 10

[WheelComplex Yin]

Hi ports,

Thanks for m*aintained devel/cpuflags.*

And I found that it is still using gcc as default CC even in FreeBSD
11-CURRENT which default CC is clang.

Here is a small patch to put in *devel/cpuflags/files, **patch
devel/cpuflags default to use clang as CC when FreeBSD version >= 10*

*Hope this can help FreeBSD better.*

*Best Regard,*

*wheelcomplex yin*