Build the OpenMP runtime in docker based builds

LLVM's OpenMP implementation only supports Windows on x86, and it
requires a MASM-compatible assembler to build it. Build UASM
for this purpose, for now. (UASM isn't available packaged in
debian/ubuntu repos.) In the future, llvm-ml should be able
to handle it instead, but it's not currently mature enough.

Not including it in the regular build-all.sh for now, as UASM
isn't generally available.

wrappers/uasm-wrapper.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+#
+# Copyright (c) 2020 Martin Storsjo
+#
+# Permission to use, copy, modify, and/or distribute this software for any
+# purpose with or without fee is hereby granted, provided that the above
+# copyright notice and this permission notice appear in all copies.
+#
+# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+ARGS=""
+while [ $# -gt 0 ]; do
+    a="$1"
+    case $a in
+    /safeseh|/coff|/c|/Fo)
+        a="-${a#/}"
+        ;;
+    esac
+    ARGS="$ARGS $a"
+    shift
+done
+
+uasm $ARGS