linux-user/elfload: Latch errno before cleanup in elf_core_dump

linux-user/elfload: Latch errno before cleanup in elf_core_dump

On the off-chance that one of the cleanup functions changes
errno, latch the errno that we want to return beforehand.

Flush errno to 0 upon success, rather than at the beginning.
No need to avoid negation of 0.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>