S390: Allow "v" constraint for long double math_opt_barrier and math_force_eval with GCC 11.

Starting with GCC 11, long double values can also be processed in vector
registers if build with -march >= z14.  Then GCC defines the
__LONG_DOUBLE_VX__ macro.

FYI: GCC commit "IBM Z: Introduce __LONG_DOUBLE_VX__ macro"
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=f47df2af313d2ce7f9149149010a142c2237beda