core: Fix byte sign extensions for architectures with default -funsigned-char

Architectures like armel, armhf, arm64, avr32, powerpc, s390 and s390x use a
unsigned type definition for char. Therefore, the macro for sign extension is
just a nop instruction there. Being more specific on these arches about the
signedness avoids this problem.
Revert "front: Commented out R4300 emulation preference until it's ready."

No further problems could be noticed and therefore enabling it again.

This reverts commit 6f9fec67f37aa3976a81d007f2f74fe69664e810.