soc/amd/non_car/memlayout_x86.ld: Top align the code

This does the following:
- Top align the bootblock so that the only the memory needed gets used.
  This might slightly reduce the time the PSP needs to decompress the
  bootblock in memory
- Use a memory directive to assert that the 16bit code is inside the top
  64K segment
- Use the program counter less. While the BDF linker is happy about
  running the program counter backwards, LLD is not. There is no
  downside to this.
- Use a symbol rather that the program counter for sections. LLD gets
  confused when (.) is used along with '<': it places the section at the
  start of the memory region, rather than at the program counter. Using
  a variable name works around this.
- Use a 'last_byte' section to make sure the first instruction is at
  0xfff0. Both the BDF and the LLD linkers seems to work well with this
  code

TEST: Both BFD and LLD are able to link the bootblock

Change-Id: I18bdf262f9c358aa01795b11efcb863686edc79c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81433
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

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