[PGO]: Eliminate calls to __llvm_profile_register_function for Linux.

On Linux, the profile runtime can use __start_SECTNAME and __stop_SECTNAME
symbols defined by the linker to locate the start and end location of
a named section (with C name). This eliminates the need for instrumented
binary to call __llvm_profile_register_function during start-up time.

llvm-svn: 250200