Always check crc in open_debug_file for now. Bug #338791 followup.

Revision r14464 made it so that debug alt files could be found by their
build-id or their (relative) file path. Debug alt files are matched using
the given build-id, but by crc. Calculating the full CRC is costly, but
currently still needed to avoid misidentifying the main file as debug
file. Slightly more efficient would be to use fstat to check we aren't
actually opening the main file under any other name (but that only works
for local DiImages). Or we could check that the file being opened actually
has at least one .debug* section. But this change was the minimal patch
to make things work as before.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14474 a5019735-40e9-0310-863c-91ae7b9d1cf9