Fine-tune compiler flags

Per default, GCC enables -fdelete-null-pointer-checks, which is
harmful and hence we should disable it:
  "Assume that programs cannot safely dereference null pointers, and
   that no code or data element resides there."

We want to be careful with our stack usage, hence enable
-fconserve-stack:
  "Attempt to minimize stack usage. The compiler will attempt to use
   less stack space, even if that makes the program slower."

Change-Id: I74eac2b07c986553f79898a2f2e57bbead4223f8
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/20883
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Makefile.inc

@@ -363,6 +363,7 @@ CFLAGS_common += -Wwrite-strings -Wredundant-decls -Wno-trigraphs
 CFLAGS_common += -Wstrict-aliasing -Wshadow -Wdate-time
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -ffunction-sections -fdata-sections -fno-pie
+CFLAGS_common += -fno-delete-null-pointer-checks -fconserve-stack
 
 ADAFLAGS_common += -gnatg -gnatp
 ADAFLAGS_common += -Wuninitialized -Wall -Werror