arc: libgloss: Use long calls attribute for exit_halt

arc: libgloss: Use long calls attribute for exit_halt

According to GCC documentation for ARC, a function marked with
the long_call attribute is always called using register-indirect
jump-and-link instructions, thereby enabling the called function
to be placed anywhere within the 32-bit address space.

exit_halt function is provided by crt0.S and it's used
in nsim-syscalls.c. Thus, we want to ensure that a linkage
issue will not arise.

Signed-off-by: Claudiu Zissulescu <claziss@gmail.com>