Since Go1.8, different types of GC mark workers were annotated and the
annotation strings were recorded during StartTrace. This change fixes
two issues around the use of traceString from StartTrace here.

1) "failed to parse trace: no consistent ordering of events possible"

This issue is a result of a missing 'batch' event entry. For efficient
tracing, tracer maintains system allocated buffers and once a buffer
is full, it is Flushed out for writing. Moreover, tracing assumes all
the records in the same buffer (batch) are already ordered and implements
more optimization in encoding and defers the completing order
reconstruction till the trace parsing time. Thus, when a Flush happens
and a new buffer is used, the new buffer should contain an event to
indicate the start of a new batch. Before this CL, the batch entry was
written only by traceEvent only when the buffer position is 0 and
wasn't written when flush occurs during traceString.

This CL fixes it by moving the batch entry write to the traceFlush.

2) crash during tracing due to invalid memory access, or during parsing
due to duplicate string entries

This issue is a result of memory allocation during traceString calls.
Execution tracer traces some memory allocation activities. Before this
CL, traceString took the buffer address (*traceBuf) and mutated the buffer.
If memory tracing occurs in the meantime from the same P, the allocation
tracing (traceEvent) will take the same buffer address through the pointer
to the buffer address (**traceBuf), and mutate the buffer.

As a result, one of the followings can happen:
 - the allocation record is overwritten by the following trace string
   record (data loss)
 - if buffer flush occurs during the allocation tracing, traceString
   will attempt to write the string record to the old buffer and
   eventually causes invalid memory access crash.
 - or flush on the same buffer can occur twice (once from the memory
   allocation, and once from the string record write), and in this case
   the trace can contain the same data twice and the parse will complain
   about duplicate string record entries.

This CL fixes the second issue by making the traceString take
**traceBuf (*traceBufPtr).

Change-Id: I24f629758625b38e1916fbfc7d7be6ea210586af
Reviewed-on: https://go-review.googlesource.com/50873
Run-TryBot: Austin Clements <austin@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>