Use the new DATAR mechanism so str-literal init on a top-level
mutable `let` lands in .data and links cleanly.
emit_lets, when it sees `let s: str = "lit"` (non-empty strlit),
emits:
DATAW s(SB),"<8 zero placeholder><8 LE bytes of len>"
DATAR s+0(SB),<strlit_label>(SB)
The linker patches the placeholder with the strlit's runtime VA at
program load time, so `s.ptr` reads as the real pointer and `s.len`
as the literal length. A new let_pre_intern pass scans top-level
lets ahead of emit_data so the strlit gets a DATA row in the same
.s file; running emit_lets after emit_data instead would have
flipped the (DATA strlits, DATAW lets) section order in the .s and
broken byte-identity with the wwstage cgen.
The wwstage cgen still emits the zero-init shape for str lets,
which only matters if the wwstage is asked to compile source that
uses str-literal init. None of the selfhost combined sources do
that today, so test 994 / 990 stay green. The selfhost mirror for
DATAR + DATAW + this w6c branch is a follow-up.
630_let_global gains two fixtures: a length-readback and a first-
byte readback through the patched ptr.
Hojun-Cho
003f707618