Fix tests which expose ldbl -> _Float128 redirects

The ldbl redirects for ieee128 have some jagged edges when
inspecting and manipulating symbols directly.

e.g asprintf is unconditionally redirected to __asprintfieee128
thus any tests relying on GCC's redirect behavior will encounter
problems if they inspect the symbol names too closely.

I've mitigated tests which expose the limitations of the
ldbl -> f128 redirects by giving them knowledge about the
redirected symbol names.

Hopefully there isn't much user code which depends on this
implementation specific behavior.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>