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/mock/Makefile.mk

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
-head.o-y += head.c
-
 libc-y += virtual.c
 
 CFLAGS += -Wno-address-of-packed-member

payloads/libpayload/arch/mock/head.c

@@ -1,3 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-
-/* This file is empty on purpose. It should not be used. */