Designation of r10 for TLS (P15) 2017-08-18 - Stafford Horne

In the Linux kernel and GCC r10 is already being used for TLS, so add it to the spec.

Changes

Update section 16.2.1 Register Usage usage of R10 from ‘Callee-saved register’ to ‘Thread Local Storage’.

Additions

In the assigned roles tables of section 16.2.1 Register Usage explain:

| R10 [TLS] | *Thread Local Storage* used to locate the thread local
              storage structure.  This mechanism, as explained in the GCC
              manual, allows variables to be "allocated such that there is
              one instance of the variable per extant thread".