cargo: go back to target-cpu=cortex-m4
Appears to give better code.
Test by matthusagh.
@@ -5,7 +5,7 @@ rustflags = [
# The target (below) defaults to cortex-m4
# There currently are two different options to go beyond that:
# 1. cortex-m7 has the right flags and instructions (FPU) but no instruction schedule yet
- "-C", "target-cpu=cortex-m7",
+# "-C", "target-cpu=cortex-m7",
# 2. cortex-m4 with the additional fpv5 instructions and a potentially
# better-than-nothing instruction schedule
"-C", "target-feature=+fp-armv8d16",
