[flang] cmake fix for FLANG_RUNTIME_F128_MATH_LIB build (#93054)

Remove an extra '-D' in the call to add_compile_definitions. The extra
-D causes -D-DFLANG_RUNTIME_F128_MATH_LIB=libquadmath to appear in
compile options which results in a compilation failure. This occurs only
on systems using cmake versions 3.22.1 or 3.24.3. Using cmake 3.28.1
seems to automatically remove the duplicate -D.

Fixes https://github.com/llvm/llvm-project/pull/81971