This option allows selecting the TLS size in the local exec TLS model,

which is the default TLS model for non-PIC objects. This allows large/
many thread local variables or a compact/fast code in an executable.

Specification is same as that of GCC. For example, the code model
option precedes the TLS size option.

TLS access models other than local-exec are not changed. It means
supoort of the large code model is only in the local exec TLS model.

Patch By KAWASHIMA Takahiro (kawashima-fj <t-kawashima@fujitsu.com>)
Reviewers: dmgreen, mstorsjo, t.p.northover, peter.smith, ostannard
Reviewd By: peter.smith
Committed by: peter.smith

Differential Revision: https://reviews.llvm.org/D71688