Fix parallel builds (sysprog21#423)

When adding the -j flag to make for faster compilation speeds, it led
to the following error:

src/riscv.h:20:10: fatal error: softfloat/softfloat.h: No such file or directory
   20 | #include "softfloat/softfloat.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

This was due to incorrect dependency settings, where compilation of the
source code started before completing the git submodule update, leading
to the error. Add appropriate dependency handling to ensure that
compilation of the softfloat library is completed before compiling the
source code.

Author: visitorckw

Makefile

@@ -175,6 +175,10 @@ OBJS := \
 OBJS := $(addprefix $(OUT)/, $(OBJS))
 deps := $(OBJS:%.o=%.o.d)
 
+ifeq ($(call has, EXT_F), 1)
+$(OBJS): $(SOFTFLOAT_LIB)
+endif
+
 $(OUT)/%.o: src/%.c $(deps_emcc)
 	$(VECHO) "  CC\t$@\n"
 	$(Q)$(CC) -o $@ $(CFLAGS) $(CFLAGS_emcc) -c -MMD -MF $@.d $<