Merge symbol tables. (Not fully ELF conformant)

Also, remove that nameptr member from symbols. It just *had* to cause
trouble.

Symbols are simply appended to the target's symbol table, which means
that LOCAL symbols are not inserted at the beginning and we are thus
ignoring an ELF spec. Might change that in the future, it's good enough
for now and it's sure not to break anything with the old symbols.

The code currently assumes that the target *has* a symbol table. We'll
have to fix that, and also remove undefined and duplicate symbols.
Basic x86-64 support, not very usable in practice

Turns out that x86-64 aligns LOAD segments to 0x200000 bytes.
Given that the mapping starts at 0x400000, there is not much room to
expand... so there will be a need to add additional LOAD headers.