automate positioning of tls_index in elf-loader

[jtracey]

The tls_index field in the VdlFile elf-loader struct corresponds to the l_tls_modid field of the link_map struct of glibc. While it isn't part of the documented glibc ABI, in practice, it gets used by gdb. Right now, we're keeping them at the same offset by hand, measuring how far in it is in glibc's implementation and moving fields around in ours so that it ends up in the same spot. If glibc ever changes this (which, as technically part of the internal ABI, they could ostensibly do whenever they want), we would have to move it as well, and deal with supporting multiple positions from the multiple glibc versions. Since gdb would have to do this as well, it's not extremely likely, but it would be nice to have a longer-term solution to this problem.