[libc++/abi] Replace uses of _NOEXCEPT in src/ by noexcept

We always build the libraries in a Standard mode that supports noexcept,
so there's no need to use the _NOEXCEPT macro.

Differential Revision: https://reviews.llvm.org/D97700

GitOrigin-RevId: 5601305fb30bf585c397e62d2d3a8a712e47d70b
diff --git a/src/thread.cpp b/src/thread.cpp
index 5959d8b..8d49e2f 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -70,7 +70,7 @@
 }
 
 unsigned
-thread::hardware_concurrency() _NOEXCEPT
+thread::hardware_concurrency() noexcept
 {
 #if defined(_SC_NPROCESSORS_ONLN)
     long result = sysconf(_SC_NPROCESSORS_ONLN);