Reland "Experimental ICU runtime linking"

This is a reland of f4aab0584dc844b1fa38e299ff1a9c1922c06a99

Original change's description:
> Experimental ICU runtime linking
>
> Introduce a build config option where ICU symbols are resolved at
> runtime, against existing system libs.
>
> Change-Id: I2325537438de0063fcc4a7c0f8411764cf550f09
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/431037
> Commit-Queue: Florin Malita <fmalita@chromium.org>
> Commit-Queue: Florin Malita <fmalita@google.com>
> Reviewed-by: Ben Wagner <bungeman@google.com>

Change-Id: I6f67a5eb842743d2486ccc4519e2d8cb9fac3458
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/433277
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
diff --git a/modules/skunicode/src/SkUnicode_icu.h b/modules/skunicode/src/SkUnicode_icu.h
new file mode 100644
index 0000000..ece8abd
--- /dev/null
+++ b/modules/skunicode/src/SkUnicode_icu.h
@@ -0,0 +1,61 @@
+/*
+ * Copyright 2021 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkUnicode_icu_DEFINED
+#define SkUnicode_icu_DEFINED
+
+#include <unicode/ubidi.h>
+#include <unicode/ubrk.h>
+#include <unicode/uscript.h>
+#include <unicode/ustring.h>
+#include <unicode/utext.h>
+#include <unicode/utypes.h>
+
+#include "include/private/SkTemplates.h"
+
+#define SKICU_EMIT_FUNCS              \
+    SKICU_FUNC(u_errorName)           \
+    SKICU_FUNC(u_getIntPropertyValue) \
+    SKICU_FUNC(u_iscntrl)             \
+    SKICU_FUNC(u_isspace)             \
+    SKICU_FUNC(u_isWhitespace)        \
+    SKICU_FUNC(ubidi_close)           \
+    SKICU_FUNC(ubidi_getLength)       \
+    SKICU_FUNC(ubidi_getLevelAt)      \
+    SKICU_FUNC(ubidi_openSized)       \
+    SKICU_FUNC(ubidi_reorderVisual)   \
+    SKICU_FUNC(ubidi_setPara)         \
+    SKICU_FUNC(ubrk_close)            \
+    SKICU_FUNC(ubrk_current)          \
+    SKICU_FUNC(ubrk_first)            \
+    SKICU_FUNC(ubrk_following)        \
+    SKICU_FUNC(ubrk_getRuleStatus)    \
+    SKICU_FUNC(ubrk_next)             \
+    SKICU_FUNC(ubrk_open)             \
+    SKICU_FUNC(ubrk_preceding)        \
+    SKICU_FUNC(ubrk_setText)          \
+    SKICU_FUNC(ubrk_setUText)         \
+    SKICU_FUNC(uloc_getDefault)       \
+    SKICU_FUNC(uscript_getScript)     \
+    SKICU_FUNC(utext_close)           \
+    SKICU_FUNC(utext_openUChars)      \
+    SKICU_FUNC(utext_openUTF8)        \
+
+#define SKICU_FUNC(funcname) decltype(funcname)* f_##funcname;
+struct SkICULib {
+    SKICU_EMIT_FUNCS
+
+    // ubrk_clone added as draft in ICU69 and Android API 31 (first ICU NDK).
+    // ubrk_safeClone deprecated in ICU69 and not exposed by Android.
+    UBreakIterator* (*f_ubrk_clone_)(const UBreakIterator*, UErrorCode*);
+    UBreakIterator* (*f_ubrk_safeClone_)(const UBreakIterator*, void*, int32_t*, UErrorCode*);
+};
+#undef SKICU_FUNC
+
+// Platform/config specific ICU factory.
+std::unique_ptr<SkICULib> SkLoadICULib();
+
+#endif // SkUnicode_icu_DEFINED