arch/x86: Clean up bootblock assembly

We have identical gdtptr16 and gdtptr. The reference in
gdtptr_offset calculation is not accounted for when
considering --gc-sections, so to support linking
gdt_init.S separately add dummy use of gdtptr symbol.

Realmode execution already accessed gdt that was located
outside [_start16bit,_estart16bit] region. Remove latter
symbol as the former was not really a start of region,
but entry point symbol.

With the romcc bootblock solution, entry32.inc may have
been linked into romstage before, but the !ENV_BOOTBLOCK
case seems obsolete now.

Change-Id: I0a3f6aeb217ca4e38b936b8c9ec8b0b69732cbb9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47964
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/arch/x86/bootblock.ld

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
-gdtptr16_offset = gdtptr16 & 0xffff;
+gdtptr_offset = gdtptr & 0xffff;
 nullidt_offset = nullidt & 0xffff;
 
 /* Symbol _start16bit must be aligned to 4kB to start AP CPUs with