[flang][runtime] Fixed include directories for Float128Math files. (#108466)

When building FortranRuntime out of tree on aarch64, the list of include
directories for the files comming from FortranFloat128MathILib
has to contain flang/runtime. I did this via
INTERFACE_INCLUDE_DIRECTORIES property.

Author: vzakhari

flang/runtime/CMakeLists.txt

@@ -251,6 +251,13 @@ if (NOT TARGET FortranFloat128Math)
       APPEND PROPERTY COMPILE_DEFINITIONS
       ${f128_defs}
       )
+    get_target_property(f128_include_dirs
+      FortranFloat128MathILib INTERFACE_INCLUDE_DIRECTORIES
+      )
+    set_property(SOURCE ${f128_sources}
+      APPEND PROPERTY INCLUDE_DIRECTORIES
+      ${f128_include_dirs}
+      )
     list(APPEND sources ${f128_sources})
   endif()
 endif()

flang/runtime/Float128Math/CMakeLists.txt

@@ -69,6 +69,9 @@ set(sources
 
 include_directories(AFTER "${CMAKE_CURRENT_SOURCE_DIR}/..")
 add_library(FortranFloat128MathILib INTERFACE)
+target_include_directories(FortranFloat128MathILib INTERFACE
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  )
 
 if (FLANG_RUNTIME_F128_MATH_LIB)
   if (${FLANG_RUNTIME_F128_MATH_LIB} STREQUAL "libquadmath")