syn: Fix Hash::new_from_array warning in non-bpf targets

[acheroncrypto]

Problem

Compiling anchor-lang in non-bpf targets e.g. anchor-cli results in #[warn(dead_code)] warning after #2682

warning: associated function `new_from_array` is never used
  --> /~/anchor/lang/syn/src/hash.rs:82:18
   |
76 | impl Hash {
   | --------- associated function in this implementation
...
82 |     pub const fn new_from_array(hash_array: [u8; HASH_BYTES]) -> Self {
   |                  ^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `anchor-syn` (lib) generated 1 warning

Summary of changes

Only include Hash::new_from_array function when target_arch = "bpf".

[vercel]

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