Add getFunction calls for endianness test function signatures

[leonm1]

These missing functions (added through the new function call types in #525) result in the following linker error when compiling against NullVM:

ld: error: undefined symbol: proxy_wasm::NullPlugin::getFunction(std::__u::basic_string_view<char, std::__u::char_traits<char>>, std::__u::function<proxy_wasm::Word (proxy_wasm::ContextBase*, proxy_wasm::Word, unsigned long, float, double)>*)
>>> referenced by null_plugin.cc
>>>               blaze-out/k8-fastbuild/_objs/null_lib/null_plugin.pic.o:(vtable for proxy_wasm::NullPlugin)

ld: error: undefined symbol: proxy_wasm::NullPlugin::getFunction(std::__u::basic_string_view<char, std::__u::char_traits<char>>, std::__u::function<unsigned long (proxy_wasm::ContextBase*, float)>*)
>>> referenced by null_plugin.cc
>>>               blaze-out/k8-fastbuild/_objs/null_lib/null_plugin.pic.o:(vtable for proxy_wasm::NullPlugin)

ld: error: undefined symbol: proxy_wasm::NullPlugin::getFunction(std::__u::basic_string_view<char, std::__u::char_traits<char>>, std::__u::function<float (proxy_wasm::ContextBase*, float, float)>*)
>>> referenced by null_plugin.cc
>>>               blaze-out/k8-fastbuild/_objs/null_lib/null_plugin.pic.o:(vtable for proxy_wasm::NullPlugin)

ld: error: undefined symbol: proxy_wasm::NullPlugin::getFunction(std::__u::basic_string_view<char, std::__u::char_traits<char>>, std::__u::function<double (proxy_wasm::ContextBase*, float, float, float)>*)
>>> referenced by null_plugin.cc
>>>               blaze-out/k8-fastbuild/_objs/null_lib/null_plugin.pic.o:(vtable for proxy_wasm::NullPlugin)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[PiotrSikora]

Could we figure out a test to fix / detect this in this repo? It's the reason for the majority of build errors.

[leonm1]

Could we figure out a test to fix / detect this in this repo? It's the reason for the majority of build errors.

I plan to do so.