* Sema: implement comptime bitcast of f80 with integer-like types
  bitwise rather than taking a round trip through memory layout.
* Type: introduce `isAbiInt`.
* Value: comptime memory write of f80 writes 0 bytes for padding
  instead of leaving the memory uninitialized.
* Value: floatReadFromMemory has a more general implementation, checking
  the endianness rather than checking for specific architectures.

This fixes behavior test failures occurring on MIPS.