Create only one section for a name in LinkerScript.

Previously, we created two or more output sections if there are
input sections with the same name but with different attributes.
That is a wrong behavior. This patch fixes the issue.

One thing we need to do is to merge output section attributes.
Currently, we create an output section based on the first input
section's attributes. This may make a wrong output section
attributes. What we need to do is to bitwise-OR attributes.
We'll do it in a follow-up patch.

llvm-svn: 278461