printf: improve support of printing multi-byte values of characters

[jtracey]

Based on #7048.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[jtracey]

I was worried because, unlike #7048, this doesn't get the printf-mb GNU test to pass, but on closer investigation, the test was only passing due to a coincidental bug that made it pass, despite failing in the general case.

The problem is the same described here: #7048 (comment)
To see it in practice, run printf '%s' $(printf '\x61\xC3\xA1\xC0') and cargo run printf '%s' $(printf '\x61\xC3\xA1\xC0'), where non-cargo printf is GNU's, and the repo is checked out at 8f4c286. You'll get aá� and aáÀ, respectively. Proper multibyte support won't work until #6812 (or a successor) gets merged.

[jtracey]

To be clear for posterity, the version in this PR will look correct and show aá�, but GNU's is actually passing the 0xC0 byte to the terminal, which is in turn rendering it with �. It's easier to see the remaining problem, the one we need #6812 to solve, if you redirect the output to a hex editor or the like.