base: increase the GDB buffer size dynamically

base: increase the GDB buffer size dynamically

The size was not large enough for the 'G' packet on aarch64, which the
client sends to set registers.

This would lead to the stub not to be able to find the end of the input
packet and keep waiting forever.

Change-Id: Icb149f15a6c769371ebcb6ec5fbebc6170c31fc6
Reviewed-on: https://gem5-review.googlesource.com/c/14497
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>