[Driver] Add -L and -l support.

Summary:
This patch adds support for -L and -l to the ld driver. It also implements
library lookup for .a files.

Reviewers: shankarke, kledzik

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D365
[ELF] Replace local dynamic tls access with direct access.
[ELF] Implement GOTPCREL for defined atoms.
[ELF][Layout] Provide a proper way to get the TLS segment size.
[ELF][x86-64] Handle PLT32 relocations to IFUNC.