BUILD: tools: rely on __ELF__ not USE_DL to enable use of dladdr()

BUILD: tools: rely on __ELF__ not USE_DL to enable use of dladdr()

The approach was wrong. USE_DL is for the makefile to know if it's required
to append -ldl at link time. Some platforms do not need it (and in fact do
not have it) yet they have a working dladdr(). The real condition is related
to ELF. Given that due to Lua, all platforms that require -ldl already have
USE_DL set, let's replace USE_DL with __ELF__ here and consider that dladdr
is always needed on ELF, which basically is already the case.

(cherry picked from commit 109201fc5c64ea1ae98e8f75f6890efa88d981bf)
Signed-off-by: Willy Tarreau <w@1wt.eu>