starkernel: fix lidt() inline-asm constraint hazard, land -O2

FABRIC.md item 4.5d Finding 4: lidt()'s inline asm used a register-only
("r") constraint on the idtr pointer, never telling GCC the asm
dereferences the pointee. At -O2 this let the compiler treat the
256-entry idt[] population loop and idtr_desc's field writes as dead
stores and eliminate them entirely, loading IDTR from uninitialized
stack instead of the real table -- a #GP on the first APIC timer tick
that happened to land on garbage. Same bug class as the earlier
muldiv64 fix (b43e51a): an inline-asm constraint too weak for what the
asm actually touches, invisible at -O0, live at -O2.

Fixed by switching to a memory operand ("m"(*idtr_desc)), matching how
Linux's own load_idt() is written. aarch64/riscv64 checked for the same
pattern -- neither has it, both install their vector/trap tables
entirely in hand-written .S.

Verified: all three architectures boot clean to ok>, POST Failed: 0,
identical dict-hashes across all three under -O2, zero new warnings
vs an -O0 baseline (amd64 3040/3040, aarch64 3041/3041 serial,
riscv64 3037/3037). -O2/-U_FORTIFY_SOURCE landed permanently in
COMMON_CFLAGS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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