Namespace `STACK_SIZE` in various assembly files

[mkannwischer]

STACK_SIZE should be MLD_STACK_SIZE to avoid clashing with symbols in consuming libraries.
It currentl occurs invarious places, e.g., in

mldsa-native/dev/aarch64_opt/src/rej_uniform_eta4_asm.S

Line 15 in 8ec5b2c

#define STACK_SIZE (2*MLDSA_N + 64)