soc/cavium/cn81xx/bootblock_custom.S: Specify arch

This fixes assembling with clang which complains about fpu instructions.

TEST: BUILD_TIMELESS=1 remains the same.

Change-Id: I175b8e749fafde5fb7ffb8101fc0dc892d9b4e0d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74539
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/cavium/cn81xx/bootblock_custom.S

@@ -4,7 +4,11 @@
 #include <arch/asm.h>
 #include <soc/addressmap.h>
 
+	.arch		armv8-a+fp
+
+
 ENTRY(_start)
+
     .org 0
      /**
      * According to the reference manual the first instruction is fetched from