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[libc++] Use runtime rather then compile-time glibc version check

Description

[libc++] Use runtime rather then compile-time glibc version check

glibc supports versioning, so it's possible to build against older
version and run against newer version. This is sometimes relied on
in practice, e.g. in Fuchsia build we build against older sysroot
(equivalent to Ubuntu Trusty) to cover the broadest possible range
of host systems, but that doesn't necessarily match the system that
binary is going to run on which may have newer version, in which case
the compile test used in curr_symbol is going to fail. Using runtime
check is more reliable.

Differential Revision: https://reviews.llvm.org/D56702