[PGO] [Coverage] put covmap into note section with no 'alloc flag' (Linux)

Coverage mapping data is not referenced by runtime, and they won't be dumped
into profile data. There is no need to allocate memory for covmap sections.
A good side effect of this change is that the coverage map data won't be mistakenly
 garbage collected by the linker (for Gold linker only, BFD linker has an issue where the a bug is filed).
Tested with clang build with instrumentation and -fcoverage-mapping and linker GC. The size of
 covmap section is ~17.6M so the text segment size will be reduced by this amount with this change.

llvm-svn: 257781