libpayload: Inject head.S into libc, remove separate class

Integrate head.S directly into libc and remove all instances of head.o.

* Drop 'separate class' entry for head.S.
* Drop special treament for head.o inside lpgcc.
* Change the .text in `x86/head.S` to `.section .text._entry`.
* Drop arch/mock/head.c, initially added as a dummy file.

Change-Id: I156d781908fcc38d455bbf9f2c29e5ab95c7775a
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82478
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/arch/x86/head.S

@@ -29,7 +29,7 @@
 
 	.code32
 	.global _entry
-	.text
+	.section .text._entry
 	.align 4
 
 /*