gdb/testsuite: Add relative versus absolute LD_LIBRARY_PATH test

gdb/testsuite: Add relative versus absolute LD_LIBRARY_PATH test

At one time, circa 2006, there was a bug, which was presumably fixed
without adding a test case:

    If you provided some relative path to the shared library, such as
    with

	export LD_LIBRARY_PATH=.

    then gdb would fail to match the shared library name during the
    TLS lookup.

I think there may have been a bit more to it than is provided by that
explanation, since the test also takes care to split the debug info
into a separate file.

In any case, this commit is based on one of Red Hat's really old
local patches.  I've attempted to update it and remove a fair amount
of cruft, hopefully without losing any critical elements from the
test.

Testing on Fedora 38 (correctly) shows 1 unsupported test for
native-gdbserver and 5 PASSes for the native target as well as
native-extended-gdbserver.

In his review of v1 of this patch, Lancelot SIX observed that
'thread_local' could be used in place of '__thread' in the C source
files.  But it only became available via the standard in C11, so I
used additional_flags=-std=c11 for compiling both the shared object
and the main program.

Also, while testing with CC_FOR_TARGET=clang, I found that
'additional_flags=-Wl,-soname=${binsharedbase}' caused clang
to complain that this linker flag was unused when compiling
the source file, so I moved this linker option to 'ldflags='.

My testing for this v2 patch shows the same results as with v1,
but I've done additional testing with CC_FOR_TARGET=clang as
well.  The results are the same as when gcc is used.

Co-Authored-by: Jan Kratochvil <jan@jankratochvil.net>
Reviewed-By: Lancelot Six <lancelot.six@amd.com>