Makefile.uk.musl.exit: Patch for case insensitive file systems

Makefile.uk.musl.exit: Patch for case insensitive file systems

Add `exit` variant to _Exit.c to prevent symbol conflicts. This
is necessary since some file systems are case insensitive, and it could
otherwise lead to a conflict with the object resulted from _exit.c

Signed-off-by: Eduard-Florin Mihailescu <mihailescu.eduard@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #70