[LLD] [MinGW] Sync --thinlto-cache-dir option details with ELF (#77010)

Disallow using the form with a separate argument,
"--thinlto-cache-dir dir", allow only the one with equals,
"--thintlo-cache-dir=dir". This is the only form that actually was
tested when this was added in
f794808b, and matches the ELF side,
where only the form with an equals is supported (and this was also the
case at the time when this option was added to the MinGW linker).