Use fwrite to print whatever the subcommand wrote

Subcommands can write things like UTF-16, which some terminals can
understand.  printf() will interpret the null bytes as the end of the
string.

In particular, MSVC's assert() will print wide characters by default,
and I can't find a way to disable it, leading to clang assertion
failures looking like:
  FAILED: ...clang.exe ...
  Aninja: build stopped: subcommand failed.

With this fix, I get the desired:
  FAILED: ...clang.exe ...
  Assertion failed: SymbolMap...
  ninja: build stopped: subcommand failed.
Merge pull request #610 from rnk/wstr

Use fwrite to print whatever the subcommand wrote