Remove wrong asm constraint modifier

From GCC documentation (for GCC 5 and above):
- ‘+’ means that this operand is both read and written by the instruction.
- Only read-only operands can use ‘%’.

Thus, ‘+’ and ‘%’ cannot be used at the same time.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
Signed-off-by: Ralph Giles <giles@thaumas.net>