dtrace: fix generation of v8 constants on freebsd

Every constant is certainly 4 bytes now, but freebsd's objdump utility
prints out odd byte sequences (5-bytes, 6-bytes and even 9-bytes long)
for v8's data section. We can safely ignore all upper bytes, because all
constants that we're using are just `int`s. Since on all supported
platforms `int` is 32bit long (and anyway v8's constants are 32bit too),
we ignore all higher bits if they were read.
gyp: fix build with dtrace support on FreeBSD

Fix undefined reference to `gelf_getsym`... and other undefined symbols
from libelf, by adding `-lelf` to linker options on FreeBSD.