[compiler-rt][Fuchsia] Use map name if provided

Prior to this, any allocations coming from the SizeClassAllocator would just
have the vmo name "SizeClassAllocator". This will instead use the string passed
to the Map functions if provided which provide slightly more info on what the
vmo is used for. That is, is it for region data, metadata, or freearray space.

Differential Revision: https://reviews.llvm.org/D158537