v2 libdl support for base image TLS variables

[chrisj at rtems.org]

These patches add support to rtems.git and rtems-tools.git to 
provide access to TLS variables in the base from loaded code.
The changes contain the ground work to resolving #4920 which is 
a blocker for the release of 6.

This change is required due to the regression in libdl due to the
change in newlib to use TLS variables for reenterant data.

The rtems.git patch can be applied before the rtems-tools.git
patch. If the tools are updated and the libdl code is not present
the tests fail with linker errors. If the tools change is
not present the test will link but it will continue to fail.

The currently supported archs that build are:

 - aarch64
 - arm
 - powerpc
 - sparc

Test results are in #4920. The following do not have newlib TLS
support so are OK:

 - i386
 - m68k

The follow are to be completed and do not build:

 - bfin
 - lm32
 - mips
 - moxie
 - riscv

These are to be removed and so not tested or built:

 - v850  

We need to add support for these architectures if they support TLS or 
list them as not needing support like the m68k.

v2:
 - Review commment updates

Chris