contrib: Set more site.exp variables in test_installed

Add support in contrib/test_installed for more variables (via
associated command-line options to the script) that gcc/Makefile.in
can set:

* ALT_CC_UNDER_TEST, ALT_CXX_UNDER_TEST and COMPAT_OPTIONS are used in
  compat testing (against the same or a different compiler).

* The libiconv variable is used for testing iconv support for
  particular character sets, and defaults to -liconv if not set in
  site.exp, which is wrong on systems with iconv in libc; keep the
  default, but add an option to override this.

  Note that the dg-require-iconv testing is currently bogus in a cross
  environment, and this patch does nothing to address that.  The tests
  using dg-require-iconv actually care about character set support on
  the *host*, for character conversions carried out in the compiler,
  and the libiconv setting put in site.exp by gcc/Makefile.in is a
  *host* library setting.  But dg-require-iconv /
  check_iconv_available tests availability when compiling, linking and
  executing for the *target*.  If the host and target have close
  enough to the same OS, this may work by accident, but otherwise it
  will incorrectly enable / disable these tests based on target
  information (but using a libiconv setting designed for the host)
  when it should be based on host information.

	* test_installed (--with-alt-cc=, --with-alt-cxx=)
	(--with-compat-options=, --with-libiconv=): New options.

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