amd64 crt1: Explicitly use a PLT entry for main in the PIC case.

This ensures GNU as generates a R_X86_64_PLT32 relocation instead of
R_X86_64_32.

Reviewed by: kib
Fixes: c969310c992a csu: Implement _start using as to satisfy unwinders on x86_64
Differential Revision: https://reviews.freebsd.org/D41101