Change in coreboot[master]: arch/x86,cpu/x86: Disable the %gs and %fs segments

[Eric Peers (Code Review)]

Attention is currently required from: Raul Rangel, Nico Huber, Julius Werner, Arthur Heymans, Felix Held.
Eric Peers has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/57860 )

Change subject: arch/x86,cpu/x86: Disable the %gs and %fs segments
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Patch Set 4:

(1 comment)

File src/arch/x86/c_start.S:

https://review.coreboot.org/c/coreboot/+/57860/comment/0924a5c5_583e8160 
PS4, Line 42: 	xor	%eax, %eax /* zero out the gs and fs segment index */
probably want to comment that you are effectively reserving these for use at a later time.

or a test to check the sourcecode files to ensure no one else touches these regs?