AES-GCM small ARM asm: add back implementation

[SparkiDev]

Description

Implementation of GCM mult with length for ARM asm and small GCM was added to armv8-aes.c but got lost when code pulled back to aes.c.

Fixes zd#20854

Testing

./configure --disable-shared LDFLAGS=--static --host=armv7m CC=arm-linux-gnueabi-gcc --enable-armasm --enable-aesgcm=small
./configure --disable-shared LDFLAGS=--static --host=armv7a CC=arm-linux-gnueabi-gcc --enable-armasm --enable-aesgcm=small

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[devin-ai-integration]

🛟 Devin Lifeguard found 2 likely issues in this PR

• no-void-functions snippet: Change static void GCM_gmult_len(...) to static int GCM_gmult_len(...) and return 0 on success (or an error code on failure); update the #define GCM_GMULT_LEN macro and any callers to handle the returned status.
• prefer-constant-time snippet: Replace “if (y & 0x80) xorbuf(Z, V, AES_BLOCK_SIZE);” with a mask-based constant-time XOR (e.g., generate a byte-wise mask = (-(y >> 7)) and xorbuf_masked(Z, V, mask)) so the same operations are executed regardless of bit value.

@SparkiDev
please take a look at the above issues which Devin flagged. Devin will not fix these issues automatically.

[SparkiDev]

retest this please

ABI:
git init /tmp/workspace/PRB-ABI-test-v2 # timeout=10
ERROR: Unable to tear down: Channel "hudson.remoting.Channel@2b07a1cb:wolf-linux-cloud-node-9yms60": Remote call on wolf-linux-cloud-node-9yms60 failed. The channel is closing down or has closed down