Call the helper function "check_shared_lib_support" to ensure -shared
is enabled before launching ld-shared, ld-elfweak and ld-elfvers.
This allows to catch custom targets explicitly disabling it.
ld/ChangeLog:
* testsuite/ld-elfvers/vers.exp: Call check_shared_lib_support.
* testsuite/ld-elfweak/elfweak.exp: Likewise.
* testsuite/ld-shared/shared.exp: Likewise.