* src/bytecode.c (exec_byte_code):
Disable the -Wclobbered warning for GCC locally as the old work-around
doesn't appear to be effective for variables with explicitly assigned
registers.  This also allows us to remove that work-around which
improves the code somewhat.

GCC seems to have difficulty allocating important global interpreter
variables in registers; telling it which ones to use for 'top' and 'pc'
makes a big difference and seems to ease pressure enough for it to
deal with other variables as well.

We do it for AMD64 and ARM64.  Clang doesn't seem to need these directives.
It does result in -Wclobbered warnings that seem difficult to silence.

* src/bytecode.c (BC_REG_TOP, BC_REG_PC): New.
(exec_byte_code): Use them.

* src/bytecode.c (exec_byte_code):
Re-type op from int to ptrdiff_t, which avoids some useless conversions.
Reduce its use by using local variables for intra-block use,
and another variable (arg) where it doesn't need to be alive across
instruction dispatch.  We also eliminate it where performance doesn't
matter by re-fetching it from the instruction stream.

All this should help the register allocator.

This struct cannot live in MPS-managed memory because it's required
for scanning the bytecode stack.

* src/bytecode.c (free_bc_thread) [MPS]: Free bytecode structure.
(Finternal_stack_stats, exec_byte_code):
* src/igc.c (scan_bc): Adjust.
(root_create_bc): Save 'bc' pointer.
* src/lisp.h (get_act_rec, set_act_rec):
* src/thread.c (mark_one_thread, finalize_one_thread): Adjust.
(Fmake_thread) [MPS]:
(init_threads) [MPS]: Allocate bytecode structure.
* src/thread.h (struct thread_state): Turn 'bc' member into a
pointer (MPS) or single-member array.