Avoid modifying the object string table when patching USDT probes.

dtrace converts pairs of consecutive underscores in a probe name to dashes.
When dtrace -G processes relocations corresponding to USDT probe sites, it
performs this conversion on the corresponding symbol names prior to looking
up the resulting probe names in the USDT provider definition. However, in
so doing it would actually modify the input object's string table, which
breaks the string suffix merging done by recent binutils. Because we don't
care about the symbol name once the probe site is recorded, just perform the
probe lookup using a temporary copy.

Reported by: hrs, swills
MFC after: 3 weeks