Files containing symbols with unicode characters in their name might not
be supported on some specific platforms. For example, the Solaris
assembler lacks support for UTF-8 characters.
```
Assembler: mangle.d
"/var/tmp//cci9q2Sc.s", line 115 : Syntax error
Near line: " movzbl test_эльфийские_письмена_9, %eax"
Assembler: testmodule.d
"/var/tmp//ccBtixAd.s", line 3 : Syntax error
Near line: " .globl _D3run17unicode_06_哪里6哪里FiZi"
Assembler: ufcs.d
"/var/tmp//ccodJ7Ib.s", line 6662 : Syntax error
Near line: " .globl _D4ufcs6α8503FiZv"
```
Add a new directive which can be used by other testsuite runners as a
hint to selectively disable the test if they know they can't compile
this ahead of time.
