csu: Find the main pointer through the GOT

Use the Global Offset Table to find the location of main in crt1. With
lld the old code would point to main@plt, however ld.bfd fails to link
when main is in a shared library.

Fix this by using the GOT address to find main as it works with both
lld and bfd.

Reviewed by: jrtc27
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45259

(cherry picked from commit 53120fbb68952b7d620c2c0e1cf05c5017fc1b27)