For GCC, newlib combined tree, newlib build-tree testing, use standard search paths

For example, for GCC/GCN target (AMD GPUs), target libraries are built
individually per supported hardware ISA ('-march=[...]').  Testing such a
toolchain via, for example:

    $ make RUNTESTFLAGS='--target_board=[...]/-march=gfx90a' check[...]

... does work fine for all 'check-gcc-[...]' as well as GCC-provided target
libraries, 'check-target-[...]'.  Just for 'check-target-newlib', for the
example above, not the '-march=gfx90a' newlib libraries are linked in, but
instead always the default ones, which results in link FAILure.  This is cured
simply by skipping use of 'newlib/testsuite/lib/flags.exp', so that the
standard search paths as determined by GCC, DejaGnu are used for newlib, too.

newlib/testsuite/lib/flags.exp

@@ -4,6 +4,13 @@
 # is freely granted, provided that this notice is preserved.
 #
 
+if [info exists env(XGCC_FLAGS_FOR_TARGET)] {
+    verbose "GCC, newlib combined tree, build-tree testing; using standard search paths"
+    # ... instead of the search paths built here, based on 'objdir' as set in
+    # newlib's 'site.exp', which always points to the default multilib.
+    return
+}
+
 # flags.exp: overrides the dejagnu versions of libgloss_link_flags,
 # newlib_link_flags, and newlib_include_flags.