Android reserves specific TLS slots to store thread specific
values. Given that the Context object gets queried on every
call we leverage Android's ASM code to improve the speed of
this operation.

TLS_SLOT_OPENGL is an unused slot in Android and using that
in combination with the ASM code,rather than using the
pthread API, allows angle to store and retrieve thread specific
context object much more efficiently.

Bug: angleproject:4717
Change-Id: I27a117fe82e62407e01c8c372918b866aaea9ee5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2231883
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>