[ELF] Don't fall back to .text for e_entry

We have the rule to simulate
(https://sourceware.org/binutils/docs/ld/Entry-Point.html),
but the behavior is questionable
(https://sourceware.org/pipermail/binutils/2021-September/117929.html).

gold doesn't fall back to .text.
The behavior is unlikely relied by projects (there is even a warning for
executable links), so let's just delete this fallback path.

Reviewed By: jhenderson, peter.smith

Differential Revision: https://reviews.llvm.org/D110014