Use the "gcc" feature in simd.prf

Matches the compiler capabilities better and will catch all GCC-like
compilers (including Clang, LLVM and Intel CC on Unix).

Task-number: QTBUG-38544
Change-Id: I102966d307a4e167b6dcf3da08359e656f3af45e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Testlib: Fix minor leak and crash when comparing lists

1) toString does not track the string it returns, thus it has to be
deleted by the caller
2) on some platforms vsnprintf crashes if a null string is in the
va_list

Change-Id: Iecf94e93d3a2ddf4186ee20de1f495f2f92dcc60
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>