[libcxx] [test] XFAIL the get/put long_double_ru_RU tests on Glibc < 2.27

Those older versions used a different monetary decimal separator.
To avoid unnecessary churn to support that, just XFAIL the test
on those older versions. (Up until
df1e43c4, the whole test was XFAILed
on all versions of glibc.)

Differential Revision: https://reviews.llvm.org/D120979