cgen: load the full header for a wwstage global-dot slice field

let x: []T = g.buf on a module-global struct dispatched only str
fields to the 3-word header load; a []T field fell to the scalar
tail (ptr word only), so len/cap read stale registers and the shape
was byteid-divergent against cstage's TY_STR||TY_SLICE arm (#263).

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1229;
+def corpuscount: i32 = 1230;
 def errorcount: i32 = 314;
 def compilecount: i32 = 12;
-def runcount: i32 = 137;
+def runcount: i32 = 138;
 def runexitcount: i32 = 766;
-def nativecount: i32 = 2458;
-def corpushash: str = "9d1db4b4bdde1b7e2b1f2e2ae5ca6c6b91b0162df585fd877021d27ec6863d89";
+def nativecount: i32 = 2460;
+def corpushash: str = "fd04059b05a19ae63f1bec3edaefedf1d8ce9a505302a153434457d0fb9b9eca";
 
 type directive = enum i32 {
 	ERROR = 0,