MIPS: Boot: Compressed: Remove -fstack-protector from CFLAGS

When building with -fstack-protector, gcc emits the __stack_chk_guard and
__stack_chk_fail symbols to check for stack stability. These symbols are
defined in vmlinux but the generated vmlinux.bin that is used to create
the compressed vmlinuz image has no symbol table so the linker can't find
these symbols during the final linking phase.  As a result of which, we
need either to redefine these symbols just for the compressed image or drop
the -fstack-protector option when building the compressed image. This patch
implements the latter of two options.

Fixes the following linking problem:

dbg.c:(.text+0x7c): undefined reference to `__stack_chk_guard'
dbg.c:(.text+0x80): undefined reference to `__stack_chk_guard'
dbg.c:(.text+0xd4): undefined reference to `__stack_chk_guard'
dbg.c:(.text+0xec): undefined reference to `__stack_chk_fail'

[ralf@linux-mips.org: I'm applying this before the patch that actually adds
stack protector support for MIPS.  This means, it will not be possible
to trigger above error message with any commit from the tree but rather
they are what one would hit without this commit.]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/5575/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/compressed/Makefile

@@ -18,6 +18,8 @@ BOOT_HEAP_SIZE := 0x400000
 # Disable Function Tracer
 KBUILD_CFLAGS := $(shell echo $(KBUILD_CFLAGS) | sed -e "s/-pg//")
 
+KBUILD_CFLAGS := $(filter-out -fstack-protector, $(KBUILD_CFLAGS))
+
 KBUILD_CFLAGS := $(LINUXINCLUDE) $(KBUILD_CFLAGS) -D__KERNEL__ \
 	-DBOOT_HEAP_SIZE=$(BOOT_HEAP_SIZE) -D"VMLINUX_LOAD_ADDRESS_ULL=$(VMLINUX_LOAD_ADDRESS)ull"