BUILDBOT: Fix AmigaOS4 Toolchain on New Buildbot

This toolchain seems to have a compiler which requires mpfr.so.4 and
thus the legacy library objects directory to be preloaded.

config/master.cfg

@@ -1630,6 +1630,7 @@ scumm_env_amigaos4["PATH"] = "%s/bin:%s/ppc-amigaos/bin:%s" % (scumm_root_amigao
 scumm_env_amigaos4["CXX"] = "ccache ppc-amigaos-g++"
 scumm_env_amigaos4["LDFLAGS"] = "-athread=native"
 scumm_env_amigaos4["PKG_CONFIG_LIBDIR"] = "%s/ppc-amigaos/lib/pkgconfig" % scumm_root_amigaos4
+scumm_env_amigaos4["LD_LIBRARY_PATH"] = "/opt/toolchains/legacy-shared-objects"
 
 p_master = {
 	"configureargs": [