pw_boot_armv7m: Add barriers for enabling the FPU
pw_boot_armv7m: Add barriers for enabling the FPU

The barriers (as recommended per the STM32 programming manual) ensure
that the FPU is correctly enabled before continuing and potentially
executing any FPU instructions, however rare that may be during startup.

Change-Id: I846c87c5fd72ac297ed6556e57cb7ca672d6d83a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13360
Commit-Queue: Ian Tessier <14ae4d305acf6965026a00efcd86d5bd850f12ea@google.com>
Commit-Queue: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>

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