[X86] Treat __ehdr_start as large (#79884)

The __ehdr_start symbol is added by the linker and points to the ELF
file headers, which can be very far away from text. Treat it as a large
symbol under the medium/large code models. Performance to access
__ehdr_start is almost certainly not important.

There are a couple of other symbols that the linker adds [1], but this
is the most relevant one that may be far away from text.

[1]
https://github.com/llvm/llvm-project/blob/547c395b279a93357082ac06cf3e8fe37ebfc3fe/lld/ELF/Writer.cpp#L226