[LSan] Add "use_ld_allocations" flag to disable old way of DTLS handling.

This flag allows to disable old way of determining dynamic TLS by
filtering out allocations from dynamic linker. This will be eventually
superseded by __tls_get_addr interceptor (see r257785), after we:
1) Test it in several supported environments
2) Deal with existing problems (currently we can't find a pointer to
  DTV which is calloc()-ed in pthread_create).

llvm-svn: 257789