runtime(asm): add basic indent support

closes: #14383

Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/indent/asm.vim

@@ -0,0 +1,28 @@
+" Vim indent file
+" Language:             asm
+" Maintainer:           Philip Jones <philj56@gmail.com>
+" Upstream:             https://github.com/philj56/vim-asm-indent
+" Latest Revision:      2017-07-01
+
+if exists("b:did_indent")
+  finish
+endif
+let b:did_indent = 1
+
+setlocal indentexpr=s:getAsmIndent()
+setlocal indentkeys=<:>,!^F,o,O
+
+let b:undo_ftplugin .= "indentexpr< indentkeys<"
+
+function! s:getAsmIndent()
+  let line = getline(v:lnum)
+  let ind = shiftwidth()
+
+  " If the line is a label (starts with ':' terminated keyword), 
+  " then don't indent
+  if line =~ '^\s*\k\+:'
+    let ind = 0
+  endif
+
+  return ind
+endfunction