Fix BANK(@) discrepancy within LOAD blocks (#2149)

RGBASM and RGBLINK disagreed on which section to consider `@` to be in;
we have decided that the latter is correct.

The `assume` change is known to be suspicious; for more discussion, see
https://github.com/gbdev/rgbds/pull/2149#discussion_r4106902830

test/asm/load-pc-bank.asm

@@ -0,0 +1,31 @@
+SECTION "rom", ROMX, BANK[1]
+xLabel:
+assert @ == xLabel
+static_assert BANK(@) == BANK(xLabel)
+static_assert SECTION(@) === SECTION(xLabel)
+	ds $3000, $42
+
+LOAD "ram", WRAMX, BANK[2]
+wLabel::
+assert @ == wLabel
+static_assert BANK(@) == BANK(wLabel)
+static_assert SECTION(@) === SECTION(wLabel)
+	ds $1000, $42
+
+ENDL
+
+SECTION "floating rom", ROMX
+xLabel2:
+assert @ == xLabel2
+assert BANK(@) == BANK(xLabel2)
+static_assert SECTION(@) === SECTION(xLabel2)
+	ds $3000, $42
+
+LOAD "floating ram", WRAMX
+wLabel2::
+assert @ == wLabel2
+assert BANK(@) == BANK(wLabel2)
+static_assert SECTION(@) === SECTION(wLabel2)
+	ds $1000, $42
+
+ENDL