Include linker generated symbol in default defined set

It seems like `_GLOBAL_OFFSET_TABLE_` is defined by the linker without
being present in any object file on x86_64. There are a few other
symbols that the linker automatically defines although we haven't run
into issues with them. I am including the full list I gathered from
lld's source code [1] by default.

[1] https://github.com/llvm/llvm-project/blob/177e38286cd61a7b5a968636e1f147f128dd25a2/lld/ELF/Config.h#L632

Author: daniel-grumberg

Sources/BinarySymbols/ReferencedSymbols.swift

@@ -13,7 +13,7 @@ package struct ReferencedSymbols {
     package private(set) var undefined: Set<String>
 
     package init() {
-        self.defined = []
+        self.defined = ["_GLOBAL_OFFSET_TABLE_"]
         self.undefined = []
     }