d: Fix junk in generated symbol on powerpc64-*-* [PR98921]

d: Fix junk in generated symbol on powerpc64-*-* [PR98921]

This adds a special formatter to OutBuffer to handle formatted printing
of integers, a common case.  The replacement is faster and safer.

In dmangle.c, it also gets rid of a number of problematic casts, as seen
on powerpc64 targets.

Reviewed-on: https://github.com/dlang/dmd/pull/12174

gcc/d/ChangeLog:

	PR d/98921
	* dmd/MERGE: Merge upstream dmd 5e2a81d9c.