all: add go:build lines (#23468)

accounts/keystore/watch.go

accounts/keystore/watch_fallback.go

cmd/utils/diskusage.go

cmd/utils/diskusage_openbsd.go

common/fdlimit/fdlimit_bsd.go

common/fdlimit/fdlimit_unix.go

core/mkalloc.go

crypto/blake2b/blake2bAVX2_amd64.go

crypto/blake2b/blake2b_amd64.go

crypto/blake2b/blake2b_f_fuzz.go

crypto/blake2b/blake2b_ref.go

crypto/blake2b/register.go

crypto/bls12381/arithmetic_decl.go

crypto/bls12381/arithmetic_fallback.go

crypto/bls12381/arithmetic_x86_adx.go

crypto/bls12381/arithmetic_x86_noadx.go

crypto/bn256/bn256_fast.go

crypto/bn256/bn256_slow.go

crypto/bn256/cloudflare/gfp_decl.go

crypto/bn256/cloudflare/gfp_generic.go