Generic configury support for shared libs on VxWorks

Generic configury support for shared libs on VxWorks

This change adds the configury bits to activate the build of
shared libs on VxWorks ports configured with --enable-shared,
for libraries variants where this is generally supported (rtp,
code model !large - currently not compatible with -fPIC).

Set lt_cv_deplibs_check_method in libtool.m4, so the build of
libraries know how to establish dependencies.  This is useful in
configurations such as aarch64 where proper support of LSE relies
on accurate dependency information between libstdc++ and libgcc_s
to begin with.

	* libtool.m4 (*vxworks*): When enable_shared, set dynamic_linker
	and friends for rtp !large. Assume the linker has the required
	abilities and set lt_cv_deplibs_check_method.