[ELF] Fix compilation with GCC 5

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,
present since SVN r346893:

 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>’

Differential Revision: https://reviews.llvm.org/D54569

llvm-svn: 346934