So far, `baremetal.sh --ram --stack` ran `callgrind` to extract
a call stack in an example run of ssl_client2 and ssl_server2.
This, however, needs to be complemented with per-function stack
usage to be able to extract the maximum stack usage.
This commit adds `-fstack-usage` to the CFLAGS used when building
the library in `baremetal.sh --ram` runs, which generates *.su
files indicating the stack usage of individual functions.
