Fix profiling handler to get the correct g for the m being profiled.
Store a pointer to the TLS slot holding g in the thread's m. This
enables the profiling handler to get the current g for the thread,
even if the thread is executing external code or system code.

Updates #26148
Signed-off-by: Jordan Rhee <jordanrh@microsoft.com>

Change-Id: Ie061284c12341c76c7d96cc0c2d5bac969230829
Reviewed-on: https://go-review.googlesource.com/c/153718
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>