BUILD: tools: unbreak resolve_sym_name() on non-GNU platforms

BUILD: tools: unbreak resolve_sym_name() on non-GNU platforms

resolve_sym_name() doesn't build when USE_DL is set on non-GNU platforms
because "Elf(W)" isn't defined. Since it's only used for dladdr1(), let's
refactor all this so that we can completely ifdef out that part on other
platforms. Now we have a separate function to perform the call depending
on the platform and it only returns the size when available.

(cherry picked from commit 9133e48f2a6fa9deccfe02798dcb2f2abd26ed9e)
Signed-off-by: Willy Tarreau <w@1wt.eu>