Remove indirection with calling JNU_NewStringPlatform

It's a follow-up to JDK-8232624.

JNU_NewStringPlatform is called via NewStringPlatform alias.
The NewStringPlatform alias is declared without JNICALL which allows looking it up with the plain name on 32 bit Windows.

JNU_NewStringPlatform is declared with JNICALL, and JNICALL modifier cannot be removed. So another fix is needed to drop NewStringPlatform as an indirect call to JNU_NewStringPlatform.

The relevant quote from discussion on the mailing list [1]:
If some or even just one platform needs a special,undecorated alias for this type of internal dll_lookup to work, it seems prudent to add it back in a way that doesn't add it back (along with a possibly non-free indirection) to every other platform.

[1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2019-October/062985.html