Move mb_string functions into runtime-common #5251

Job	Run time
macos/clang++/c++17	5m 26s
macos/clang++/c++17	5m 26s
macos/clang++/c++17	4m 10s
buster/g++/c++17/asan=off/ubsan=off	17m 1s
buster/g++/c++17/asan=off/ubsan=off	17m 1s
buster/g++/c++17/asan=off/ubsan=off	16m 56s
focal/clang++/c++17/asan=off/ubsan=on	18m 49s
focal/clang++/c++17/asan=off/ubsan=on	18m 49s
focal/clang++/c++17/asan=off/ubsan=on	19m 17s
focal/g++-10/c++20/asan=on/ubsan=off	18m 45s
focal/g++-10/c++20/asan=on/ubsan=off	18m 45s
focal/g++-10/c++20/asan=on/ubsan=off	20m 24s
jammy/g++/c++20/asan=on/ubsan=off	18m 27s
jammy/g++/c++20/asan=on/ubsan=off	18m 47s
jammy/g++/c++20/asan=on/ubsan=off	18m 21s
	3h 56m 24s

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