openssl: add gcc-8 -ffile-prefix-map filter

gcc-8 switch -ffile-prefix-map helps a lot with reproducible build paths
in the resulting binaries.

Ref: https://reproducible-builds.org/docs/build-path/
Signed-off-by: Paul Spooren <mail@aparcar.org>
[refactored into separate commit]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

package/libs/openssl/patches/120-strip-cflags-from-binary.patch

@@ -17,7 +17,7 @@ index 2c619c62e8..893128345a 100644
  
  DEPEND[cversion.o]=buildinf.h
 -GENERATE[buildinf.h]=../util/mkbuildinf.pl "$(CC) $(LIB_CFLAGS) $(CPPFLAGS_Q)" "$(PLATFORM)"
-+GENERATE[buildinf.h]=../util/mkbuildinf.pl "$(filter-out -I% -iremap% -fmacro-prefix-map%,$(CC) $(LIB_CFLAGS) $(CPPFLAGS_Q))" "$(PLATFORM)"
++GENERATE[buildinf.h]=../util/mkbuildinf.pl "$(filter-out -I% -iremap% -fmacro-prefix-map% -ffile-prefix-map%,$(CC) $(LIB_CFLAGS) $(CPPFLAGS_Q))" "$(PLATFORM)"
  DEPEND[buildinf.h]=../configdata.pm
  
  GENERATE[uplink-x86.s]=../ms/uplink-x86.pl $(PERLASM_SCHEME)