LD/testsuite: Enable execution with remote targets

LD/testsuite: Enable execution with remote targets

Lift target execution limitation from `run_ld_link_exec_tests' procedure
and also handle remote targets, albeit in a restricted way, as follows.

Numerous test cases have been written such that they refer to a shared
library using a relative path, precluding the use of the runtime library
search path for the dependency to be found at run time.  Consequently a
test affected has to be run on the target such that the shared library
required is present at the same relative path rooted at the program's
current working directory.

Additionally any shared library dependencies are not handled across the
test suite in an organised way, so test invocations only have names of
direct dependencies buried within the linker options supplied.

Therefore assert that the linker test directory is located at the same
absolute path both on the test host and the test target, typically via
sharing the directory over NFS, and use the DejaGNU `remotedir' feature
to start execution on the target with that location as the test case's
current working directory.