Link mksnapshot with libatomic on x86

Clang-12 on x86 emits atomic builtins

Fixes
| module-compiler.cc:(.text._ZN2v88internal4wasm12_GLOBAL__N_123ExecuteCompilationUnitsERKSt10shared_ptrINS2_22BackgroundCompileTokenEEPNS0_8CountersEiNS2_19CompileBaselineOnlyE+0x558): un
defined reference to `__atomic_load'

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/tools/v8_gypfiles/v8.gyp
+++ b/tools/v8_gypfiles/v8.gyp
@@ -1436,6 +1436,7 @@
     {
       'target_name': 'mksnapshot',
       'type': 'executable',
+      'libraries': [ '-latomic' ],
       'dependencies': [
         'v8_base_without_compiler',
         'v8_compiler_for_mksnapshot',