[gdb/testsuite] Use shlib gdb_compile option in gdb.base/skip-solib.exp

[gdb/testsuite] Use shlib gdb_compile option in gdb.base/skip-solib.exp

In test-case gdb.base/skip-solib.exp the linking against a shared library is
done manually:
...
if {[gdb_compile "${binfile_main}.o" "${binfile_main}" executable \
        [list debug "additional_flags=-L$testobjdir" \
             "additional_flags=-l${test}" \
             "ldflags=-Wl,-rpath=$testobjdir"]] != ""} {
...

Instead, use the shlib gdb_compile option such that we simply have:
...
        [list debug shlib=$binfile_lib]] != ""} {
...

Tested on x86_64-linux.