elf: Support lld-style link map for librtld.map

GNU ld and gold's -Map include a line like:

  path/to/build/libc_pic.a(check_fds.os)

lld -Map does not have the archive member list, but we can still derive the
members from the following output

             VMA              LMA     Size Align Out     In      Symbol
...
           1a1c0            1a1c0       e2    16         path/to/build/libc_pic.a(check_fds.os):(.text)