elf_loader: Use only basename of PT_INTERP when loading via manifest file
elf_loader: Use only basename of PT_INTERP when loading via manifest file

When elf_loader is used in the browser, it gets no arguments and
always loads a file called "main.nexe".  In this case, the full
pathname in PT_INTERP is not what it should open.  Just the basename
(nondirectory part) of that pathname is what appears in the manifest
file as a dictionary key.

BUG= https://code.google.com/p/chromium/issues/detail?id=543385
TEST= trybots
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/1411923003 .

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client