This commit provides a universal TLS_BASE virtual register on ARM, similar to as exists
on x86. Depending on the precise configuration this virtual register maps to a different
register
* aarch64: TPIDRURW is used for the TLS_BASE and is already declared and being saved
and restored on context switches, so this just adds TLS_BASE as an alias of it
* armv6: Has no hardware register for use for a TLS_BASE, and so the virtual register
gets stored into the globals frame
* armv7+: TPIDURO is used for TLS_BASE and so the restore paths are modified to load
TLS_BASE into it
7d7f338b2b