build: remove unused libatomic on ppc64, s390x

The library is not necessary for ppc64 or s390x. It does no harm with
some linkers, but devtoolset-6 creates runtime dependencies on all link
libraries, even unused ones.

Fixes: nodejs#27377
Fixes: nodejs#29718

tools/v8_gypfiles/v8.gyp

@@ -795,7 +795,7 @@
         }],
         # Platforms that don't have Compare-And-Swap (CAS) support need to link atomic library
         # to implement atomic memory access
-        ['v8_current_cpu in ["mips", "mipsel", "mips64", "mips64el", "ppc", "ppc64", "s390x"]', {
+        ['v8_current_cpu in ["mips", "mipsel", "mips64", "mips64el", "ppc"]', {
           'link_settings': {
             'libraries': ['-latomic', ],
           },