toolchain: fix gcc bug 99140 condition

Gcc bug 99140 exhibits with gcc versions:
- up to 7.x
- 9.x
- 10.x
and doesn't show up with gcc versions:
- 8.x
- 11.x
then moving BR2_TOOLCHAIN_GCC_AT_LEAST_9 to BR2_TOOLCHAIN_GCC_AT_LEAST_10
makes gcc version 9 set as working but it's not. So let's back substitute
BR2_TOOLCHAIN_GCC_AT_LEAST_10 with BR2_TOOLCHAIN_GCC_AT_LEAST_9.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>