[AIX][TLS][clang] Add -maix-small-local-exec-tls clang option.

This patch adds the clang portion of an AIX-specific option to inform
the compiler that it can use a faster access sequence for the local-exec
TLS model (formally named aix-small-local-exec-tls).

This patch only adds the frontend portion of the option, building upon:

Backend portion of the option (D156203)
Backend patch that utilizes this option to actually produce the faster access sequence (D155600)

Differential Revision: https://reviews.llvm.org/D155544