Rollup merge of #104001 - Ayush1325:custom-entry, r=bjorn3

Improve generating Custom entry function

This commit is aimed at making compiler-generated entry functions (Basically just C `main` right now) more generic so other targets can do similar things for custom entry. This was initially implemented as part of https://github.com/rust-lang/rust/pull/100316.

Currently, this moves the entry function name and Call convention to the target spec.

Signed-off-by: Ayush Singh <ayushsingh1325@gmail.com>

diff --cc compiler/rustc_codegen_llvm/src/context.rs
Simple merge

diff --cc compiler/rustc_target/src/abi/call/mod.rs
Simple merge

diff --cc compiler/rustc_target/src/spec/mod.rs
Simple merge