Stop creating covmap as note section on ELF

covmap needs to created as non allocatable, but not with
SHT_NOTE. The latter was needed to workaround a problem
of BFD linker with gc, which is no longer needed. (A more
proper longer term fix requires changing FE driver to force
referencing the section using linker script).

Differential Revision: http://reviews.llvm.org/D17309

llvm-svn: 261228