arch/riscv: Avoid gcc11 replacing memset implementation with memset call

SBI comes with its own memset implementation (under a slightly
different name) that gcc11 "helpfully" tries to replace with a call
to memset(). Since we don't provide a memset, the linker isn't happy,
so prevent gcc from doing that.

Change-Id: I3459a519d46a123f873306000b8b2261bd64e0c3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56429
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/arch/riscv/Makefile.inc

@@ -171,7 +171,7 @@ $(OPENSBI_TARGET): $(obj)/config.h | $(OPENSBI_SOURCE)
 	mkdir -p $(OPENSBI_BUILD)
 	$(MAKE) \
 		-C "$(OPENSBI_SOURCE)" \
-		CC="$(CC_ramstage)" \
+		CC="$(CC_ramstage) -fno-builtin" \
 		LD="$(LD_ramstage)" \
 		OBJCOPY="$(OBJCOPY_ramstage)" \
 		AR="$(AR_ramstage)" \