As noted in #20978, the linker would previously handle overflowed
relocations by creating a jump island. While this is fine in the case of
code symbols, it's very much not okay in the case of data symbols. To
fix this we must keep track of whether each symbol is code or data and
relocate them appropriately. This patch takes the first step in this
direction, adding a symbol type field to the linker's symbol table. It
doesn't yet change relocation behavior to take advantage of this
knowledge.

Fixes #20978.