Turn _idn2_punycode_encode, _idn2_punycode_decode into compat symbols

These internal symbols should not be used by applications, but were
previously exported.  Contrary to our expectations, outright removal
is not possible due to some use in old GNUTLS versions.

The aliases are required because internal hidden references turn the
target symbol into a hidden symbol.  It would have been possible to
preserve the internal API using another __asm__ alias, but this commit
renames the call sites instead, for simplicity.

The -export-symbols-regex argument in lib/Makefile.am is not adjusted
because targets which do not support version scripts probably do not
have stringent backwards compatibility requirements.

Fixes libidn/libidn2#74.