cmd/compile: TestEmptyDwarfRanges failing on Plan 9

[0intro]

TestEmptyDwarfRanges has been added in CL 94816 and is failing on Plan 9. This is happening because Plan 9 executables don't have a DWARF symbol table.

--- FAIL: TestEmptyDwarfRanges (0.64s)
	scope_test.go:461: no DWARF data in Plan 9 file
FAIL
FAIL	cmd/compile/internal/gc	165.627s

See https://build.golang.org/log/d1594a762c6c9c32559aa6d2b85a1d7af18fe69d

[gopherbot]

Change https://golang.org/cl/98275 mentions this issue: cmd/compile: skip TestEmptyDwarfRanges on Plan 9