Expand the initialism UCRT in rustc book

This commit expands the initialism UCRT on first use in the rusct book
chapter on the *-windows-gnullvm platform support, making the chapter
easier to understand. Microsoft, the originator of this technology in
Windows, expands it to Universal C Runtime[1]. The same expansion is
added to the chapter.

[1] https://learn.microsoft.com/en-us/cpp/porting/upgrade-your-code-to-the-universal-crt

Author: miikkas

src/doc/rustc/src/platform-support/windows-gnullvm.md

@@ -2,7 +2,7 @@
 
 **Tier: 2 (with host tools)**
 
-Windows targets similar to `*-windows-gnu` but using UCRT as the runtime and various LLVM tools/libraries instead of
+Windows targets similar to `*-windows-gnu` but using Universal C Runtime (UCRT) as the runtime and various LLVM tools/libraries instead of
 GCC/Binutils.
 
 Target triples available so far: