toolchain,build: prefer -ffile-prefix-map for gcc-8+

-ffile-prefix-map=OLD=NEW is an alias for both -fdebug-prefix-map and
-fmacro-prefix-map and is available since GCC 8.

Co-Developed-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Paul Spooren <mail@aparcar.org>
[refactored into separate commit]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

rules.mk

@@ -140,11 +140,11 @@ else
   TOOLCHAIN_DIR_NAME:=toolchain-$(GNU_TARGET_NAME)
 endif
 
-ifeq ($(or $(CONFIG_EXTERNAL_TOOLCHAIN),$(CONFIG_GCC_VERSION_4_8),$(CONFIG_TARGET_uml)),)
-  ifeq ($(CONFIG_GCC_USE_EMBEDDED_PATH_REMAP),y)
-    iremap = -fmacro-prefix-map=$(1)=$(2)
-  else
+ifeq ($(or $(CONFIG_EXTERNAL_TOOLCHAIN),$(CONFIG_TARGET_uml)),)
+  ifeq ($(CONFIG_GCC_USE_IREMAP),y)
     iremap = -iremap$(1):$(2)
+  else
+    iremap = -ffile-prefix-map=$(1)=$(2)
   endif
 endif