[ELF] - Linkerscript KEEP command.

When link-time garbage collection is in use (-gc-sections), it is 
often useful to mark sections that should not be eliminated. 
This is accomplished by surrounding an input section's wildcard 
entry with KEEP(). Patch implements that command.

Differential revision: http://reviews.llvm.org/D17242

llvm-svn: 261616