gdb: make gdbarch_data_registry static

gdb: make gdbarch_data_registry static

This variable was made static in:

  6bd434d6caa4 ("gdb: make some variables static")

But I modified gdbarch.c instead of gdbarch.sh, so the change was
later reverted when gdbarch.c was re-generated.

Do it right this time.

gdb/ChangeLog:

	* gdbarch.sh (gdbarch_data_registry): Make static.
	* gdbarch.c: Re-generate.

Change-Id: I4048ba99a0cf47acd9da050934965db222fbd159