Avoid signed overflow in RGBLINK's +, -, and * (#2060)

RGBASM computes these with unsigned arithmetic and casts back, since
signed overflow is UB in C++, but RGBLINK's RPN evaluator used `int32_t`
directly, so `src/link/patch.cpp` tripped UBSan on lines 117, 121, and
124. Share the three operators through `opmath.cpp` so both evaluators
stay in step.

test/link/patch-wraparound.asm

@@ -0,0 +1,9 @@
+def fzero equs "startof(\"test\")"
+section "test", rom0
+; Signed overflow is undefined behavior, so these must wrap around instead
+dl $7fff_ffff + ({fzero} + 1)
+dl $8000_0000 - ({fzero} + 1)
+dl $7fff_ffff * ({fzero} + 3)
+
+; XXX: We rely on this landing at address $0000, which isn't *guaranteed*...
+assert startof("test") == 0