[UCRT] Fix GCC/Clang SIMD compilation

GCC and Clang need to mark functions that use SSE/AVX etc, either with a function attribute or a pragma around the function. strlen uses a template function that either uses SSE2 or AVX2. Previously the template was surrounded with pragmas to allow both SSE2 and AVX2, but that makes GCC assume that it can use AVX2 instructions even in the SSE2 version. To fix this the template instances are now build in individual compilation units for SSE2 and AVX, separate from the "dispatcher" function.
Now ucrtbase doesn't crash anymore on GCC build.

Another issue was the namespace around strnlen_mode, which has confused clang so much, that it forgot to instantiate the template code.

sdk/lib/ucrt/string/strnlen-sse2.cpp

@@ -0,0 +1,29 @@
+//
+// strnlen-sse2.cpp
+//
+//      Copyright (c) Timo Kreuzer
+//
+// Explicit template instantiations for SSE2 str(n)len code
+//
+
+#pragma GCC target("sse2")
+#define _UCRT_BUILD_SSE2
+#include "strnlen.cpp"
+
+template
+size_t __cdecl common_strnlen_simd<bounded, __crt_simd_isa::sse2, uint8_t>(
+    uint8_t const* const string,
+    size_t         const maximum_count
+    ) throw();
+
+template
+size_t __cdecl common_strnlen_simd<bounded, __crt_simd_isa::sse2, uint16_t>(
+    uint16_t const* const string,
+    size_t         const maximum_count
+    ) throw();
+
+template
+size_t __cdecl common_strnlen_simd<unbounded, __crt_simd_isa::sse2, uint16_t>(
+    uint16_t const* const string,
+    size_t         const maximum_count
+    ) throw();