lib/libc/minimal: Move sqrt/sqrtf from samples

The lmp90100_evb sample included an implementation of double sqrt, and the
on_off_level_lighting_vnd_app sample included an implementation of float
sqrtf. Move that code into minimal libc instead of requiring applications
to hand-roll their own version.

Signed-off-by: Keith Packard <keithp@keithp.com>