target/target: read_memory 64-bit bugfix

Increase "value_buf" size so it can hold a 64-bit number represented
as a string. Previous size could only hold a 32-bit number string.

Signed-off-by: Daniel Goehring <dgoehrin@os.amperecomputing.com>
Change-Id: If6fbc875236e6ddc59522fbc25db0129eb60ee27
Reviewed-on: https://review.openocd.org/c/openocd/+/7221
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>