x86_64 assembly: Don't touch %gs

With CPU_INFO_V2 enabled %gs holds the pointer to the cpu_info struct,
so don't clobber it. Backup and restore %gs where possible.
Fixes a crash in MPinit seen after calling FSP-S.

Change-Id: If9fc999b34530de5d8b6ad27b9af25fc552e9420
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59764
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/security/intel/txt/getsec_enteraccs.S

@@ -227,6 +227,10 @@ cond_clear_var_mtrrs:
 	movd	%esp, %xmm0
 	movd	%ebp, %xmm1
 
+	/* Backup %gs used by CPU_INFO_V2 */
+	movl	%gs, %eax
+	movd	%eax, %xmm2
+
 	/*
 	 * Get function arguments.
 	 * It's important to pass the exact ACM size as it's used by getsec to verify
@@ -261,6 +265,8 @@ cond_clear_var_mtrrs:
 	movl	%eax, %es
 	movl	%eax, %ss
 	movl	%eax, %fs
+	/* Restore %gs used by CPU_INFO_V2 */
+	movd	%xmm2, %eax
 	movl	%eax, %gs
 
 	/* Disable cache */