cmd: use 128-bit SHA256 & encode in base64 for content hashes
We used to use SHA1 for content hashes, but CL 402595 changed
all the “don't care” hashes to cmd/internal/notsha256 (negated SHA256).
This made object files a little bit bigger: fmt.a on my Mac laptop grows
from 910678 to 937612 bytes (+3%).
To remove that growth, truncate the hash we use for these purposes
to 128 bits (half a SHA256), and also use base64 instead of hex for
encoding it when a string form is needed. This brings fmt.a down to
901706 bytes (-1% from original, -4% from current).