picolibc: Disable LTO while building picolibc bits

The compiler requires that much of the C library be built without using LTO
so that various symbols are available for use by generated code, including
things like memset and memcpy.

Add -fno-lto when building both picolibc itself as well as the Zephyr
interface bits.

Closes: #81674

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