[sanitizer] Disable -ffunction-sections.

It breaks when a binary is linked with --gc-sections: parts of sanitizer
interface get thrown away and inaccessible from dlopen-ed libs.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200683 91177308-0d34-0410-b5e6-96231b3b80d8