Fixes to build libmono with disabled JIT. (#4421)

Fixes to build libmono with disabled JIT. (#4421)

Current mono build on (at least Windows) fails if build with disabled JIT
since the .def file used when building libmono, mono-2.0-sgen.dll, exports
a couple of symbols missing when DISABLE_JIT is defined.

Turns out that these methods have been incorrectly exported and should not
be part of the public API surface. This commit removes MONO_API for the
methods incorrectly marked:

mono_emit_native_call
mono_inst_name
mono_replace_ins

It also removes them from the .def files so they won't be exported in the
mono runtime DLL on Windows.

For mono_set_break_policy an empty stub will be added since it is considered
being a public API.

The above support is primarily used on none desktop Windows platforms
when building mono runtime module without JIT support.

This commit also fix a problem introduced in tramp-amd64.c by commit
https://github.com/mono/mono/commit/4281399ba4b7c44a30fa2c2a314a470260bb3eb1
causing it to get duplicated definitions of mono_arch_get_enter_icall_trampoline
when DISABLE_JIT has been defined.