[M68k] Mark public functions with the LLVM_EXTERNAL_VISIBILITY macro

In 0dbcb363, most most target symbols were made hidden by default
with the public ones marked with LLVM_EXTERNAL_VISIBILITY. When the
M68k target was added, this particular change was forgotten so that
external tools cannot make use of the public M68k target functions
in libLLVM.so. Thus, add the missing LLVM_EXTERNAL_VISIBILITY macro
to all public target functions in the M68k backend.

Differential Revision: https://reviews.llvm.org/D99869