Avoid the need to install when running the jit testsuite

Avoid discrepancy between the installed name of the driver as used
by libgccjit (e.g. "x86_64-unknown-linux-gnu-gcc-5.0.0") and that
within the builddir by adding a simple symlink to xgcc within the
builddir, with the install-time name.

When running the testsuite, set PATH and LIBRARY_PATH when invoking
built binaries that use libgccjit so that it can find the driver, and so
that when the driver invokes the linker, the linker can find libgcc,
libgcc_s, crtbeginS.o, etc, finding them all in the builddir via "xgcc"
via libgloss.

This avoids the need for installation when running the testsuite, whilst
avoiding "baking in" any paths into the built library.

This patch doesn't touch the documentation (I plan to do that as a
followup).

gcc/ChangeLog.jit:
	* Makefile.in (FULL_DRIVER_NAME): New variable, adapted from the
	install-driver target.  New target, a symlink within the builddir,
	linked to "xgcc", for use when running the JIT library from the
	builddir.
	(MOSTLYCLEANFILES): Add FULL_DRIVER_NAME.
	(install-driver): Use $(FULL_DRIVER_NAME) rather than spelling it
	out.
	(site.exp): Don't set "bindir", as this is no longer needed when
	running the jit testsuite.

gcc/jit/ChangeLog.jit:
	* Make-lang.in (jit): Add $(FULL_DRIVER_NAME) as a dependency, so
	that the symlink is created for testing.

	* jit-playback.c (gcc::jit::playback::context::compile): Add
	"-fno-use-linker-plugin" when invoking the driver.  Update error
	messages to talk about the "gcc driver" rather than the
	"gcc harness".  To ease troubleshooting, add error messages giving
	the driver name and PATH to the error-handling code that fires
	when the driver can't be found.

gcc/testsuite/ChangeLog.jit:
	* jit.dg/jit.exp (get_path_of_driver): New procedure.
	(jit-dg-test): Don't unsetenv GCC_EXEC_PREFIX, since jit-playback.c
	now adds -fno-use-linker-plugin to the driver cmdline sidestepping
	the builddir/installdir libtto_plugin naming issue.
	When setting up PATH so that the JIT library can invoke the driver
	by installation name, don't use the installation "bindir".
	Instead, simply use the location of xgcc as detected
	get_path_of_driver.  In addition, set up LIBRARY_PATH so that the
	linker run from inside the JIT library can locate libgcc etc when
	building the .so, pointing it at the same directory.