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/arm/Makefile.mk

@@ -29,7 +29,7 @@
 CFLAGS += -mthumb -march=armv7-a
 arm_asm_flags = -Wa,-mthumb -Wa,-mimplicit-it=always -Wa,-mno-warn-deprecated
 
-head.o-y += head.S
+libc-y += head.S
 libc-y += eabi_compat.c
 libc-y += main.c sysinfo.c
 libc-y += timer.c coreboot.c util.S
@@ -44,5 +44,4 @@ libc-$(CONFIG_LP_GPL) += memcpy.S memset.S memmove.S
 libgdb-y += gdb.c
 
 # Add other classes here when you put assembly files into them!
-head.o-S-ccopts += $(arm_asm_flags)
 libc-S-ccopts += $(arm_asm_flags)