arch/x86/bootblock_crt0.S: Leverage eax in protected mode entry

Leverage already used eax register in bootblock_protected_mode_entry.
Avoid another register ebx just for preserving eax value as it is not
needed and is not used at all after moving the value into mm0.
Allow EBX to be preserved for other usage.

Change-Id: Ia668b78f2f97cf026692f1fe63ff8a382a162474
Signed-off-by: Himanshu Sahdev <himanshusah@hcl.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35292
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/bootblock_crt0.S

@@ -47,11 +47,10 @@ bootblock_protected_mode_entry:
 	/* MMX registers required here */
 
 	/* BIST result in eax */
-	movl	%eax, %ebx
+	movd	%eax, %mm0
 
 	/* Get an early timestamp */
 	rdtsc
-	movd	%ebx, %mm0
 	movd	%eax, %mm1
 	movd	%edx, %mm2
 #endif