Allow c-style main functions

[gooncreeper]

Fixes #12569

Adds support for main functions with signatures

pub fn main() c_int
pub fn main(argc: c_int, argv: [*:null]?[*:0]c_char) c_int
pub fn main(argc: c_int, argv: [*:null]?[*:0]c_char, envp: [*:null]?[*:0]c_char) c_int

[gooncreeper]

I am unsure why the CI is failing

[alexrp]

This will need a rebase.

[gooncreeper]

I misunderstood the issue, this should only work when linking against libc.