ELF: Add +, -, *, / and & to SECTIONS linker script command.

This patch is based heavily on George Rimor's patch
http://reviews.llvm.org/D19221.

In the linker script, you can write expressions to compute addresses.
Currently we only support "+" operator. This adds a few more operators.

Since this patch adds * and /, we need to handle operator precedences
properly. I implemented that using the operator-precedence grammar.

Differential Revision: http://reviews.llvm.org/D19237

llvm-svn: 266798