use GC.malloc_atomic to allocate objects without pointers

src/base64.cr

@@ -160,7 +160,7 @@ module Base64
   # This will decode either the normal or urlsafe alphabets.
   def decode(data) : Bytes
     slice = data.to_slice
-    buf = Pointer(UInt8).malloc(decode_size(slice.size))
+    buf = GC.malloc_atomic(decode_size(slice.size)).as(UInt8*)
     appender = buf.appender
     from_base64(slice) { |byte| appender << byte }
     Slice.new(buf, appender.size.to_i32)

src/big/lib_gmp.cr

@@ -180,7 +180,7 @@ lib LibGMP
 end
 
 LibGMP.set_memory_functions(
-  ->(size) { GC.malloc(size) },
+  ->(size) { GC.malloc_atomic(size) },
   ->(ptr, old_size, new_size) { GC.realloc(ptr, new_size) },
   ->(ptr, size) { GC.free(ptr) }
 )

src/regex.cr

@@ -449,7 +449,7 @@ class Regex
     return ($~ = nil) if byte_index > str.bytesize
 
     ovector_size = (@captures + 1) * 3
-    ovector = Pointer(Int32).malloc(ovector_size)
+    ovector = GC.malloc_atomic(ovector_size).as(Int32*)
     ret = LibPCRE.exec(@re, @extra, str, str.bytesize, byte_index, (options | Options::NO_UTF8_CHECK), ovector, ovector_size)
     if ret > 0
       match = MatchData.new(self, @re, str, byte_index, ovector, @captures)