VxWorks config fixes for shared objects

VxWorks config fixes for shared objects

This strengthens the VxWorks configuration files for the support
of shared objects, which encompasses a VxWorks specific "non-static"
mode for RTPs (in addition to -static and -shared).

2020-11-06  Fred Konrad  <konrad@adacore.com>
	    Olivier Hainque  <hainque@adacore.com>

gcc/
	* config/vx-common.h: Define REAL_LIBGCC_SPEC since the
	'-non-static' option is not standard.
	* config/vxworks.h (VXWORKS_LIBGCC_SPEC): Implement the LIBGCC_SPEC
	since REAL_LIBGCC_SPEC is used now.
	(STARTFILE_PREFIX_SPEC): Use the PIC VSB when building shared libraries
	or non-static binaries.