[profile][test] Improve coverage-linkage.cpp with ld.lld --gc-sections

The __llvm_prf_names section uses SHF_GNU_RETAIN.  However, GNU ld before 2015-10
(https://sourceware.org/bugzilla/show_bug.cgi?id=19161) neither supports it nor
retains __llvm_prf_names according to __start___llvm_prf_names. So --gc-sections
does not work on such old GNU ld.

This is not a problem for gold and sufficiently new lld.