[makefile] Allow .sizes target to work with funny-named objects

[makefile] Allow .sizes target to work with funny-named objects

The bin/xxx.sizes targets examine the list of obj_ symbols in bin/xxx.tmp
to determine which objects to measure the size of. These symbols have been
normalized to C identifiers, so the result is an error message from `size'
when examining a target that includes objects that were originally named
with hyphens.

Fix by turning obj_foo_bar into $(wildcard bin/foo?bar.o) instead of
bin/foo_bar.o.

Signed-off-by: Marty Connor <mdc@etherboot.org>