[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 #81971

Author: dpalermo

flang/CMakeLists.txt

@@ -336,7 +336,7 @@ endif()
 
 if (FLANG_RUNTIME_F128_MATH_LIB)
   add_compile_definitions(
-    -DFLANG_RUNTIME_F128_MATH_LIB="${FLANG_RUNTIME_F128_MATH_LIB}"
+    FLANG_RUNTIME_F128_MATH_LIB="${FLANG_RUNTIME_F128_MATH_LIB}"
     )
 endif()