[clang] Default x86-64's medium code model -mlarge-data-threshold to 65535 (#67506)

This matches gcc.

This means that by default, under x86-64's medium code model we treat
globals < 2^16 bytes as "small data" and globals >= 2^16 bytes as "large
data".

The previous clang behavior of treating all data as "large data" can be
set with `-mlarge-data-threshold=0`.

See
https://discourse.llvm.org/t/rfc-matching-gccs-mlarge-data-threshold-for-x86-64s-medium-code-model/73727.