printf: Fix formatting hexadecimals with padding

[frendsick]

When printing hex number, with 0 padding, the extra zeros go in between 0x and the digits. We put them at the beginning:

env printf "%#06x\n" 123
0x007b
$ ./coreutils-main printf "%#06x\n" 123
000x7b

Fixes: #7510

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)