fix thread issue with PERL_GLOBAL_STRUCT

fix thread issue with PERL_GLOBAL_STRUCT

The MY_CXT subsystem allows per-thread pseudo-static data storage.
Part of the implementation for this involves each XS module being
assigned a unique index in its my_cxt_index static var when first
loaded.

Because PERL_GLOBAL_STRUCT bans any static vars, under those builds
there is instead a table which maps the MY_CXT_KEY identifying string to
index.

Unfortunately, this table was allocated per-interpreter rather than
globally, meaning if multiple threads tried to load the same XS module,
crashes could ensue.

This manifested itself in failures in
ext/XS-APItest/t/keyword_plugin_threads.t

The fix is relatively straightforward: allocate PL_my_cxt_keys globally
rather than per-interpreter.

Also record the size of this struct in a new var, PL_my_cxt_keys_size,
rather than doing double duty on PL_my_cxt_size.