mrsas module: Use MACHINE_ARCH exlcusively

TARGET_ARCH is always wrong when not used at the toplevel Makefile*, or
in something that has to be included from there. Switch to using
MACHINE_ARCH exclusively here since there's no benefit from assigning
TARGET_ARCH the value of MACHINE_ARCH and then using TARGET_ARCH (and
make TARGET_ARCH=xxx won't work).

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