avoid unnecessary copies with emplace_back()

[firewave]

This is split from similar changes for push_back() as some of those caused sizable regressions which need to be investigated first :( This is another step in preparation of applying the misc-const-correctness clang-tidy-16 fixes.

The impact is neglectable but there are no regressions.

Performing a --enable=all --inconclusive scan on mame_regtest.

With DISABLE_VALUEFLOW=1:

GCC 12 1,849,957,730 -> 1,849,441,950
Clang 14 1,901,388,783 -> 1,901,388,929

With Valueflow enabled:

Clang 14 50,981,418,024 -> 50,988,014,183
GCC 12 54,791,127,471 -> 54,794,443,556

And scanning cli with --enable=all --inconclusive -Ilib -D __GNUC__ and DISABLE_VALUEFLOW=1

GCC 12 3,216,157,747 -> 3,215,371,284
Clang 14 3,178,610,619 -> 3,178,527,042