Fix ldbl-128 / ldbl-128ibm erfcl for -Wuninitialized

The ldbl-128 and ldbl-128ibm implementations of erfcl produce
uninitialized variable warnings with -Wuninitialized because of switch
statements where in fact one of the cases will always be executed, but
the compiler does not see that these cases cover all possibilities
(and because the reasoning that it does involves inequalities on the
representation of a floating point value leading to a set of possible
values for 8.0 times that value, converted to int, it's highly
nontrivial for the compiler to see that).  This patch fixes those
warnings by converting the last case in those switch statements to a
"default" case.

Tested for powerpc and mips64.

	* sysdeps/ieee754/ldbl-128/s_erfl.c (__erfcl): Make case 9 in
	switch statement into default case.
	* sysdeps/ieee754/ldbl-128ibm/s_erfl.c (__erfcl): Likewise.