Support for general reduction kernels.

Requires coordinated change in frameworks/base.

Requires coordinated change in frameworks/compile/libbcc in order
for RsTest to run.

At present, general reduction kernels are run single-threaded.

Also: Remove dead struct field MTLaunchStructForEach::sig.

Bug: 23535724
Change-Id: Ice17ccf20a902f8a106eaa62ec071d46e3c0ad8c
22 files changed