tsan: remove indirection when selecting the active mapping

Currently there are 2 levels when selecting the active mapping:
the branchy ifdef tree + another ifdef tree in SelectMapping.
Moreover, there is an additional indirection for some platforms
via HAS_48_BIT_ADDRESS_SPACE define. This makes already complex
logic even more complex and almost impossible to read.
Remove one level of indirection and define the active mapping
in SelectMapping.

Depends on D107742.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107743