x86/efistub: Use 1:1 file:memory mapping for PE/COFF .compat section

In the Linux kernel, the following vulnerability has been resolved:

x86/efistub: Use 1:1 file:memory mapping for PE/COFF .compat section

The .compat section is a dummy PE section that contains the address of

the 32-bit entrypoint of the 64-bit kernel image if it is bootable from

32-bit firmware (i.e., CONFIG_EFI_MIXED=y)

This section is only 8 bytes in size and is only referenced from the

loader, and so it is placed at the end of the memory view of the image,

to avoid the need for padding it to 4k, which is required for sections

appearing in the middle of the image.

Unfortunately, this violates the PE/COFF spec, and even if most EFI

loaders will work correctly (including the Tianocore reference

implementation), PE loaders do exist that reject such images, on the

basis that both the file and memory views of the file contents should be

described by the section headers in a monotonically increasing manner

without leaving any gaps.

So reorganize the sections to avoid this issue. This results in a slight

padding overhead (< 4k) which can be avoided if desired by disabling

CONFIG_EFI_MIXED (which is only needed in rare cases these days)