fix #15617(fix compilation failure on -d:useMalloc --gc:none) (#17555)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

lib/system/mmdisp.nim

@@ -57,6 +57,16 @@ elif defined(gogc):
 elif (defined(nogc) or defined(gcDestructors)) and defined(useMalloc):
   include system / mm / malloc
 
+  when defined(nogc):
+    proc GC_getStatistics(): string = ""
+    proc newObj(typ: PNimType, size: int): pointer {.compilerproc.} =
+      result = alloc0(size)
+
+    proc newSeq(typ: PNimType, len: int): pointer {.compilerproc.} =
+      result = newObj(typ, align(GenericSeqSize, typ.align) + len * typ.base.size)
+      cast[PGenericSeq](result).len = len
+      cast[PGenericSeq](result).reserved = len
+
 elif defined(nogc):
   include system / mm / none
 

tests/system/tgcnone.nim

@@ -0,0 +1,6 @@
+discard """
+  matrix: "--gc:none -d:useMalloc"
+"""
+# bug #15617
+let x = 4
+doAssert x == 4