[lldb] Fix 'ptsname_r' is only available on macOS 10.13.4 or newer

A deployment target less than 10.13.4 causes an error saying that
'ptsname_r' is only available on macOS 10.13.4 or newer. The current
logic only checks if the symbol is available and doesn't account for the
deployment target. This patch fixes that by adding an availability
check.

Differential revision: https://reviews.llvm.org/D125995