fix breakage when user overrides CFLAGS on the make command line

these per-target CFLAGS adjustments are mandatory additions to the
command line for building the affected targets, not part of the
user-provided CFLAGS for tuning. my intent was always that the
variable append operations would take place after user settings, but
when a variable is set on the command line, it overrides all
definitions in the makefile, including target-specific ones.

based on patch by Szabolcs Nagy.

Makefile

@@ -94,22 +94,22 @@ crt/crt1.o crt/Scrt1.o crt/rcrt1.o src/ldso/dlstart.lo: $(wildcard arch/$(ARCH)/
 
 crt/rcrt1.o: src/ldso/dlstart.c
 
-crt/Scrt1.o crt/rcrt1.o: CFLAGS += -fPIC
+crt/Scrt1.o crt/rcrt1.o: CFLAGS_ALL += -fPIC
 
 OPTIMIZE_SRCS = $(wildcard $(OPTIMIZE_GLOBS:%=src/%))
 $(OPTIMIZE_SRCS:%.c=%.o) $(OPTIMIZE_SRCS:%.c=%.lo): CFLAGS += -O3
 
 MEMOPS_SRCS = src/string/memcpy.c src/string/memmove.c src/string/memcmp.c src/string/memset.c
-$(MEMOPS_SRCS:%.c=%.o) $(MEMOPS_SRCS:%.c=%.lo): CFLAGS += $(CFLAGS_MEMOPS)
+$(MEMOPS_SRCS:%.c=%.o) $(MEMOPS_SRCS:%.c=%.lo): CFLAGS_ALL += $(CFLAGS_MEMOPS)
 
 NOSSP_SRCS = $(wildcard crt/*.c) \
 	src/env/__libc_start_main.c src/env/__init_tls.c \
 	src/thread/__set_thread_area.c src/env/__stack_chk_fail.c \
 	src/string/memset.c src/string/memcpy.c \
 	src/ldso/dlstart.c src/ldso/dynlink.c
-$(NOSSP_SRCS:%.c=%.o) $(NOSSP_SRCS:%.c=%.lo): CFLAGS += $(CFLAGS_NOSSP)
+$(NOSSP_SRCS:%.c=%.o) $(NOSSP_SRCS:%.c=%.lo): CFLAGS_ALL += $(CFLAGS_NOSSP)
 
-$(CRT_LIBS:lib/%=crt/%): CFLAGS += -DCRT
+$(CRT_LIBS:lib/%=crt/%): CFLAGS_ALL += -DCRT
 
 # This incantation ensures that changes to any subarch asm files will
 # force the corresponding object file to be rebuilt, even if the implicit