unix: Time to build with --gc-sections.

This actually saves "only" 6K for x86_64 build, as we're still more or less
careful to #ifdef unneeded code. But relying on --gc-sections in a "lazy"
manner would allow to make #ifdef'ing less pervasive (not suggested right
away, but an option for the future).

unix/Makefile

@@ -28,7 +28,7 @@ ifdef DEBUG
 CFLAGS += -g
 COPT = -O0
 else
-COPT = -Os #-DNDEBUG
+COPT = -Os -fdata-sections -ffunction-sections #-DNDEBUG
 # _FORTIFY_SOURCE is a feature in gcc/glibc which is intended to provide extra
 # security for detecting buffer overflows. Some distros (Ubuntu at the very least)
 # have it enabled by default.
@@ -68,7 +68,7 @@ else
 # Use gcc syntax for map file
 LDFLAGS_ARCH = -Wl,-Map=$@.map,--cref
 endif
-LDFLAGS = $(LDFLAGS_MOD) $(LDFLAGS_ARCH) -lm $(LDFLAGS_EXTRA)
+LDFLAGS = $(LDFLAGS_MOD) $(LDFLAGS_ARCH) -lm $(LDFLAGS_EXTRA) -Wl,--gc-sections
 
 ifeq ($(MICROPY_FORCE_32BIT),1)
 # Note: you may need to install i386 versions of dependency packages,