[AIX][TLS] Add target attribute for -maix-small-local-exec-tls option.

This patch adds a target attribute for an AIX-specific option that
informs the compiler that it can use a faster access sequence for the
local-exec TLS model (formally named aix-small-local-exec-tls).

The Clang portion of this option is in D155544.
The initial implementation to generate the faster access sequence is in
D155600.

Differential Revision: https://reviews.llvm.org/D156203