misc: create C declarations for the _addr and _semi m5ops

misc: create C declarations for the _addr and _semi m5ops

Symbols such as m5_exit_addr are already present in the libm5.a, but were
not previously exposed in a header. This commit allows external C programs
to use those versions of the functions as well.

Change-Id: I925e3af7bd6cb23e06fb744d453153323afb9310
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36896
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>