x86/head/64: Disable stack protection for head$(BITS).o

On 64-bit, the startup_64_setup_env() function added in

  866b556e ("x86/head/64: Install startup GDT")

has stack protection enabled because of set_bringup_idt_handler().
This happens when CONFIG_STACKPROTECTOR_STRONG is enabled. It
also currently needs CONFIG_AMD_MEM_ENCRYPT enabled because then
set_bringup_idt_handler() is not an empty stub but that might change in
the future, when the other vendor adds their similar technology.

At this point, %gs is not yet initialized, and this doesn't cause a
crash only because the #PF handler from the decompressor stub is still
installed and handles the page fault.

Disable stack protection for the whole file, and do it on 32-bit as
well to avoid surprises.

 [ bp: Extend commit message with the exact explanation how it happens. ]
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/20201008191623.2881677-6-nivedita@alum.mit.edu

arch/x86/kernel/Makefile

@@ -47,6 +47,8 @@ endif
 # non-deterministic coverage.
 KCOV_INSTRUMENT		:= n
 
+CFLAGS_head$(BITS).o	+= -fno-stack-protector
+
 CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace
 
 obj-y			:= process_$(BITS).o signal.o