[ELF] - Linkerscript: implemented ADDR command.

ADDR(section)
Return the absolute address (the VMA) of the named section.

Used in the wild, eg.: https://searchcode.com/file/53617342/arch/x86/kernel/vmlinux.lds.S

Differential revision: https://reviews.llvm.org/D23913

llvm-svn: 280070