poly1305: fix building with 'arm-linux-gnueabihf-gcc-11 -O3'

* cipher/poly1305.c [HAVE_COMPATIBLE_GCC_ARM_PLATFORM_AS]
(ADD_1305_32): Reduce number of register operands.

Ubuntu 21.10 arm-linux-gnueabihf-gcc gave following error with -O3:

poly1305.c: In function '_gcry_poly1305_update_burn':
cipher/poly1305.c:425:7: error: 'asm' operand has impossible constraints

425 |       ADD_1305_32(h4, h3, h2, h1, h0, m4, m3, m2, m1, m0);
    |       ^

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>