elf: Self-dlopen failure with explict loader invocation [BZ #24900]

elf: Self-dlopen failure with explict loader invocation [BZ #24900]

In case of an explicit loader invocation, ld.so essentially performs
a dlopen call to load the main executable.  Since the pathname of
the executable is known at this point, it gets stored in the link
map.  In regular mode, the pathname is not known and "" is used
instead.

As a result, if a program calls dlopen on the pathname of the main
program, the dlopen call succeeds and returns a handle for the main
map.  This results in an unnecessary difference between glibc
testing (without --enable-hardcoded-path-in-tests) and production
usage.

This commit discards the names when building the link map in
_dl_new_object for the main executable, but it still determines
the origin at this point in case of an explict loader invocation.
The reason is that the specified pathname has to be used; the kernel
has a different notion of the main executable.