[BOLT][CMAKE] Use IN_LIST check

Summary:
Address @smeenai feedback https://reviews.llvm.org/D117061#inline-1122106:
>CMake has if(IN_LIST) now, which you can use instead of the string(FIND)

IN_LIST is available since CMake 3.3 released in 2015.

Reviewed By: smeenai

FBD33590959