fix(format): keep macro content behind a label on its line

The canonical form splits a label from the instruction that follows it,
but an identifier naming a macro may expand into any token at all: moved
to a line of its own it no longer parses, because the parser accepts a
non-mnemonic first token only behind a label.  The names #define'd in
the file now hold such content back, conservatively across the whole
file, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/testdata/fuzz/FuzzFormatExpansionRoundTrip/e6b15df9546d86e6

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define M 0\nA:M")