stm32/Makefile: Quote libgcc path so spaces are not an issue.

Fixes #3116.

ports/stm32/Makefile

@@ -124,7 +124,7 @@ endif
 
 LDFLAGS = -nostdlib -L $(LD_DIR) $(addprefix -T,$(LD_FILES)) -Map=$(@:.elf=.map) --cref
 LDFLAGS += --defsym=_estack_reserve=8
-LIBS = $(shell $(CC) $(CFLAGS) -print-libgcc-file-name)
+LIBS = "$(shell $(CC) $(CFLAGS) -print-libgcc-file-name)"
 
 # Remove uncalled code from the final image.
 CFLAGS += -fdata-sections -ffunction-sections