fix #20012 (#20013)

* replace gcc asm with __asm__ and add a test

* update test case to specify gcc or clang and not cpp

Co-authored-by: daniel <danielclarke@wearepopgun.com>

tests/compiler/tasm.nim

@@ -0,0 +1,15 @@
+proc testAsm() =
+  let src = 41
+  var dst = 0
+
+  asm """
+    mov %1, %0\n\t
+    add $1, %0
+    : "=r" (`dst`)
+    : "r" (`src`)"""
+
+  doAssert dst == 42
+
+when defined(gcc) or defined(clang) and not defined(cpp):
+  {.passc: "-std=c99".}
+  testAsm()