libc: minimal: Add 'noreturn' attribute to abort() and exit()

This aligns abort() and exit() definitions with other libc.

Without 'noreturn' attribute, compilers have to assume that we will
return from these functions which can lead to surprising errors like
'error: non-void function does not return a value'.

Signed-off-by: Patryk Duda <patrykd@google.com>