rts/Linker: Fix __dso_handle handling
Previously the linker's handling of __dso_handle was quite wrong. Not
only did we claim that __dso_handle could be NULL when statically
linking (which it can not), we didn't even implement this mislead theory
faithfully and instead resolved the symbol to a random pointer. This
lead to the failing relocations on AArch64 noted in #20493.
Here we implement __dso_handle as a dynamic linker would do,
choosing an address within the loaded object (specifically its start
address) to serve as the object's handle.
I believe this should fix the AArch64 test failure seen in !6485 (closed) .