efi/x86: Decompress at start of PE image load address

When booted via PE loader, define image_offset to hold the offset of
startup_32() from the start of the PE image, and use it as the start of
the decompression buffer.

[ mingo: Fixed the grammar in the comments. ]

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200303221205.4048668-3-nivedita@alum.mit.edu
Link: https://lore.kernel.org/r/20200308080859.21568-17-ardb@kernel.org