gdbserver: convert init_register_cache and new_register_cache into constructors

This is a refactoring that converts

  init_register_cache (struct regcache *regcache,
                       const struct target_desc *tdesc,
                       unsigned char *regbuf)

into the constructor

  regcache (const target_desc *tdesc, unsigned char *regbuf)

and converts

  new_register_cache (const struct target_desc *tdesc)

into the constructor

  regcache (const target_desc *tdesc)

Also use DISABLE_COPY_AND_ASSIGN for additional compile-time safety.

Tested by rebuilding gdbserver with '--enable-inprocess-agent=no' and
with '--enable-inprocess-agent=yes'.

Approved-By: Simon Marchi <simon.marchi@efficios.com>