[ExecutionEngine] Fix the call to DWARFContext::create

Without this patch, we pass G.getEndianness() as one of the parameters
to DWARFContext::create.  The problem is that G.getEndianness() is of:

  enum endianness {big, little, native};

whereas DWARFContext::create is expecting "bool isLittleEndian".  That
is, we are relying on an implicit conversion to convert big and little
to false and true, respectively.

When we migrate llvm::support::endianness to std::endian in future, we
can no longer rely on an implicit conversion because std::endian is
declared with "enum class".  Even if we could, the conversion would
not be guaranteed to work because, for example, libcxx defines:

  enum class endian {
    little = 0xDEAD,
    big = 0xFACE,
    :

where big and little are not boolean values.

This patch fixes the problem by properly converting G.getEndianness()
to a boolean value.