elf: resolve R_X86_64_DPTMOD64 properly

R_X86_64_DPTMOD64 may not be associated with any symbol (which is common
when the linker uses local dynamic TLS mode) in which case it should be
resovled to the index of the current module.

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>