[LLD] [ELF] Fix compilation with GCC 5

[mstorsjo]

Summary

Remove the default initializer for TrapInstr; all subclasses overwrite the defaults in their constructors anyway.

This fixes compilation errors like these, with GCC 5.4 on Ubuntu 16.04:

In file included from ../tools/lld/ELF/Arch/AArch64.cpp:12:0:
../tools/lld/ELF/Target.h:125:49: error: array must be initialized with a brace-enclosed initializer
   std::array<uint8_t, 4> TrapInstr = {0, 0, 0, 0};
                                                 ^
../tools/lld/ELF/Target.h:125:49: error: too many initializers for ‘std::array<unsigned char, 4ul>’

lld/trunk/ELF/Target.h

public:
 
   // On PPC ELF V2 abi, the first entry in the .got is the .TOC.
   unsigned GotHeaderEntriesNum = 0;
 
   bool NeedsThunks = false;
 
   // A 4-byte field corresponding to one or more trap instructions, used to pad
   // executable OutputSections.
-  std::array<uint8_t, 4> TrapInstr = {0, 0, 0, 0};
+  std::array<uint8_t, 4> TrapInstr;
 
   // If a target needs to rewrite calls to __morestack to instead call
   // __morestack_non_split when a split-stack enabled caller calls a
   // non-split-stack callee this will return true. Otherwise returns false.
   bool NeedsMoreStackNonSplit = true;
 
   virtual RelExpr adjustRelaxExpr(RelType Type, const uint8_t *Data,
                                   RelExpr Expr) const;