[ELF] - linkerscript AT keyword (in output section description) implemented.

The linker will normally set the LMA equal to the VMA. 
You can change that by using the AT keyword.
The expression lma that follows the AT keyword specifies 
the load address of the section.

Patch implements this keyword.

Differential revision: https://reviews.llvm.org/D19272

llvm-svn: 278911