more plugin testsuite issues

[Alan Modra]

Hi Dave,
  Is there a good reason why we need $LIBS in plugin.exp:58
set libs "$LIBS $HOSTING_LIBS"

I have some partial cross toolchains installed (some without even
libc).  They all fail a number of the plugin tests due to missing -ldl
and/or -lz, which are added to LIBS via configure.in AC_SEARCH_LIBS.

In fact, using HOSTING_LIBS and HOSTING_CRT0 in plugin.exp is a little
confusing.  They are really only properly defined for native binutils
(configure.host selects on $host but uses the target $CC to figure out
startup and lib names).  Perhaps plugin.exp ought to be native only?

-- 
Alan Modra
Australia Development Lab, IBM