[libc++] Fix a few warnings in system headers with GCC

This isn't fixing all of them, but at least it's making some progress.

Differential Revision: https://reviews.llvm.org/D106283

NOKEYCHECK=True
GitOrigin-RevId: 2e4755ff6058b5032b77ccca04aa6dab7db081a5
diff --git a/include/cmath b/include/cmath
index a520685c..adf83c2 100644
--- a/include/cmath
+++ b/include/cmath
@@ -621,7 +621,7 @@
 
     if (__t == 1) return __b;
     const _Fp __x = __a + __t * (__b - __a);
-    if (__t > 1 == __b > __a)
+    if ((__t > 1) == (__b > __a))
         return __b < __x ? __x : __b;
     else
         return __x < __b ? __x : __b;