[libc] Remove atomic alignment diagnostics globally (#96803)

Summary:
These warnings mean that it will lower to a libcall. Previously we just
disabled it locally, which didn't work with GCC. This patch does it
globally in the compiler options if the compiler is clang.

Author: jhuber6

libc/src/stdlib/rand.cpp

@@ -13,10 +13,6 @@
 
 namespace LIBC_NAMESPACE {
 
-// Silence warnings on targets with slow atomics.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Watomic-alignment"
-
 // An implementation of the xorshift64star pseudo random number generator. This
 // is a good general purpose generator for most non-cryptographics applications.
 LLVM_LIBC_FUNCTION(int, rand, (void)) {
@@ -33,6 +29,4 @@ LLVM_LIBC_FUNCTION(int, rand, (void)) {
   }
 }
 
-#pragma GCC diagnostic pop
-
 } // namespace LIBC_NAMESPACE

libc/src/stdlib/srand.cpp

@@ -12,14 +12,8 @@
 
 namespace LIBC_NAMESPACE {
 
-// Silence warnings on targets with slow atomics.
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Watomic-alignment"
-
 LLVM_LIBC_FUNCTION(void, srand, (unsigned int seed)) {
   rand_next.store(seed, cpp::MemoryOrder::RELAXED);
 }
 
-#pragma GCC diagnostic pop
-
 } // namespace LIBC_NAMESPACE