Replaces 'char' with 'signed char' for PowerPC

PowerPC and some other (EBCIDIC-based?) architectures assume chars are
unsigned, while x86 and most ASCII-based architectures assume char is
signed.  The decode routine assumes that chars are signed.  Not a good
assumption for portability.  It's easily fixed by sprinkling the
'signed' qualifier into base64_decode_value() where it counts.

Had to change a couple of (char)-casts to (signed char) in
base64_decode_block(), too.  This isn't the best for speed on PowerPC,
since the (signed) int returned from base64_decode_value() has to be
cast to char, then sign extended in a separate instruction.  If you need
a few cycles back, consider hacking the return type of
base64_decode_value to signed char.