SYS_OPEN is already defined in stdio.h of MinGW-w64,
therefore the compiler complains when building for w64.

Adding the prefix TARGET_ avoids that macro redefinition.
xtensa-semi.c also uses the same prefix (but mixed case macros
TARGET_SYS_xxx instead of TARGET_SYS_XXX).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Peter Maydell <peter.maydell@linaro.org>