Allow larger regblock sizes when saving tracefiles

Allow larger regblock sizes when saving tracefiles

The tracefile.c:trace_save function assumes trace_regblock_size won't
be larger than the MAX_TRACE_UPLOAD constant, used to size the buffer
which holds trace data.  This can cause buffer overruns when this is
not the case.  This patch changes this function so that the larger
size is used to size the buffer.

gdb/ChangeLog:
2018-08-06  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* tracefile.c: Include common/byte-vector.h.
	(trace_save): Change type of buf to gdb::byte_vector.  Initialize
	with trace_regblock_size if needed.  Update uses of buf.