why does libatomic need gcc15?

[Greg Troxel <gdt%lexort.com@localhost>]

I am starting pkg_rr for a number of pkg builders, and on n10-i386 I see
libatomic wants gcc15.  That seems aggressively high, and not
particularly aligned with any base gcc.  There's no comment in the
Makefile about why this version is chosen.

Perhaps, after the download finishes (it's "only" 100 MB), the build
will not take so long, and this is not worth worrying about.   But I am
surprised by this.