debug/ltrace: Fix HOST_OS and ar

HOST_OS really is the target OS. Allow setting it for configure
via an environment variable.
libltrace.a should have an index:
Allow ar to be set as an environment variable, and generate
an index in this lib.

Reported-by: "Guylhem Aznar" <crossgcc@guylhem.net>
Signed-off-by: "Titus von Boxberg" <titus@v9g.de>

 --- binutils/ld/ldmain.c

 +++ binutils/ld/ldmain.c

 @@ -296,6 +296,7 @@ main (int argc, char **argv)

    link_info.allow_undefined_version = TRUE;

    link_info.keep_memory = TRUE;

 +  link_info.new_dtags = TRUE;

    link_info.combreloc = TRUE;

    link_info.strip_discarded = TRUE;

    link_info.callbacks = &link_callbacks;