build: Don't add -latomic for clang

compiler-rt supports `__atomic_is_lock_free` since LLVM 12[0]. Requiring
`-latomic` on clang builds introduces a dependency on libgcc, which is
often undesirable on LLVM-based build environments. This change makes
sure it's never added in clang builds, so it can use the compiler-rt's
implementation, making it possible to build Node.js on Linux systems
with LLVM as the main toolchain, without having to install GCC.

compiler-rt's `__atomic_is_lock_free` works on all architectures, so
there is no need to to link libatomic for mips64, ppc, arm, riscv etc.
as well, as long as clang is used.

Similar patch was already adopted or discussed in:

* Chimera Linux
  * https://github.com/chimera-linux/cports/blob/master/contrib/nodejs/patches/no-libatomic.patch
* Gentoo (LLVM profiles)
  * https://bugs.gentoo.org/869992
  * https://bugs.gentoo.org/911340
  * gentoo/gentoo#33141

[0] llvm/llvm-project@00530de

node.gyp

@@ -508,9 +508,6 @@
           '-Wl,-bnoerrmsg',
         ],
       }],
-      ['OS=="linux" and clang==1', {
-        'libraries': ['-latomic'],
-      }],
     ],
   },
 

tools/v8_gypfiles/v8.gyp

@@ -1245,7 +1245,7 @@
         }],
         # Platforms that don't have Compare-And-Swap (CAS) support need to link atomic library
         # to implement atomic memory access
-        ['v8_current_cpu in ["mips64", "mips64el", "ppc", "arm", "riscv64", "loong64"]', {
+        ['clang==0 and v8_current_cpu in ["mips64", "mips64el", "ppc", "arm", "riscv64", "loong64"]', {
           'link_settings': {
             'libraries': ['-latomic', ],
           },