build: fix rule for building dynamic files

[vtjnash]

Previously the architecture-specific dynamic files were relying on the built-in rules alone.

[martin-frbg]

Sorry, I do not see what this would be fixing, when the built-in rule works fine (and has so for years)

[vtjnash]

I appreciate the quick review, but please reopen this. This was accidentally relying on an old-fashioned technique (not my name for it, literally that is what this feature is called https://www.gnu.org/software/make/manual/html_node/Suffix-Rules.html) that does not work reliably for many uses. In particular, the rule is only implemented when $(SUFFIX) is defined to be .o, but that means this one file breaks the build if any part of the Makefile uses a different SUFFIX value. This is a really simple fix for that.