[libc] Use rint builtin for rounding on the GPU (llvm#98345)

Summary:
Previously this went through the generic bit-twiddling implementation
instead of using the dedicated GPU instruction. This patch adds this in
to the utility, mirroring the special-casing of the x64 and aarch
targets. This results in much nicer code. The following example shows
the opencl device libs implementation on the left and the LLVM libc on
the right, https://godbolt.org/z/3ch48ccf5. The libc version is
"branchier", but the results seem similar.

Author: jhuber6

libc/src/__support/FPUtil/nearest_integer.h

@@ -17,6 +17,18 @@
 #include "x86_64/nearest_integer.h"
 #elif defined(LIBC_TARGET_ARCH_IS_AARCH64)
 #include "aarch64/nearest_integer.h"
+#elif defined(LIBC_TARGET_ARCH_IS_GPU)
+
+namespace LIBC_NAMESPACE {
+namespace fputil {
+
+LIBC_INLINE float nearest_integer(float x) { return __builtin_rintf(x); }
+
+LIBC_INLINE double nearest_integer(double x) { return __builtin_rint(x); }
+
+} // namespace fputil
+} // namespace LIBC_NAMESPACE
+
 #else
 
 namespace LIBC_NAMESPACE {