makefile: default to cargo debug build

we should define debug and release targets, but hardcoding in makefile
for now.

Signed-off-by: Tianhao Wang <shrik3@mailbox.org>

Author: shrik3

Makefile

@@ -21,7 +21,7 @@ ASMOBJFORMAT = elf64
 ASMFLAGS = -w-zeroing
 LINKER_SCRIPT = ./defs/$(ARCH)-linker.ld
 CARGO_XBUILD_TARGET = ./defs/$(ARCH)-rustubs.json
-CARGO_XBUILD_FLAGS = --release
+CARGO_XBUILD_FLAGS =
 # ---------- No need to edit below this line --------------
 # ---------- If you have to, something is wrong -----------
 LDFLAGS = -no-warn-rwx-segment -static -e startup