target/riscv: Use glib2 instead of strcasecmp/strncasecmp

This is a change in semantics. g_ascii_strcasecmp() doesn't honour
locale but strcasecmp() does. But this is OK for at least one reason:
 (1) QEMU always runs with the C locale so there's not an actual
     behaviour change here

Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Link: https://lore.kernel.org/r/20260327134401.270186-8-kkostiuk@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>