[LLD] - Improve handling of AT> linker script commands

Patch by Konstantin Schwarz!

The condition to create a new phdr must also check the usage of "AT>" 
linker script command, and create a new PT_LOAD header if a new LMARegion is used.

This fixes PR38307

Differential revision: https://reviews.llvm.org/D50052

llvm-svn: 338679