gdb.trace: Use g packet order in tfile_fetch_registers.

tfile_fetch_registers currently wrongly fetches registers using
gdb order instead of g packet order.  On x86_64 with AVX, this causes
problems with ymm*h and orig_rax registers: gdb has ymm*h first, while
g packet has orig_rax first.

gdb/ChangeLog:

	* tracefile-tfile.c (tfile_fetch_registers): Use g packet order
	instead of gdb order.

gdb/doc/ChangeLog:

	* gdb.texinfo (Trace File Format): Remove misleading information
	about register block ordering.