devel/kBuild: fix build on powerpc*

Add architecture-specific code to kHlpAssertBreakpoint() so that
the build progresses on powerpc-derived platforms.

This whole thing can probably be replaced with just

#define kHlpAssertBreakpoint __builtin_trap

but I have decided for the minimal invasive change for now.

Approved by: portmgr (build fix blanket)
Sponsored by: Raptor Computing Systems, LLC
MFH: 2026Q1
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