Faster bytecode immediate argument fetching

Faster bytecode immediate argument fetching

* src/bytecode.c (FETCH2):
Use `|` instead of `+` to combine the bytes forming a 16-bit immediate
argument so that GCC (prior to version 12) recognises the idiom and
generates a 16-bit load.  This applies for little-endian machines with
cheap unaligned accesses such as x86[-64], arm64 and power64le.

This 1-character change results in a measurable speed gain on many
kinds of Lisp code, as 16-bit immediates are used by all jump
instructions.

Clang performs this optimisation for both `+` and `|` from version 10.