toolchain/toolchain-wrapper: reproducible builds: handle debug info paths in GCC <7

In GCC8 the flag -ffile-prefix-map handles cleaning up both the __FILE__
macros and the debug info paths. In GCC7 or below we are manually
handling the __FILE__ macros, but not debug info paths. Use
-fdebug-prefix-map to clean them up. This option exists since GCC 4.3.0,
which is our minimal supported GCC version.

See for more detail: https://reproducible-builds.org/docs/build-path/

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

toolchain/toolchain-wrapper.mk

@@ -26,6 +26,7 @@ TOOLCHAIN_WRAPPER_OPTS += -Wl,--build-id=none
 ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_8),y)
 TOOLCHAIN_WRAPPER_OPTS += -ffile-prefix-map=$(BASE_DIR)=buildroot
 else
+TOOLCHAIN_WRAPPER_OPTS += -fdebug-prefix-map=$(BASE_DIR)=buildroot
 TOOLCHAIN_WRAPPER_OPTS += -D__FILE__=\"\" -D__BASE_FILE__=\"\" -Wno-builtin-macro-redefined
 endif
 ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_7),)