arm920t fix constant error in start.S

Code in cpu/arm920t/start.S will die with a compilation error if
CONFIG_STACKSIZE + CFG_MALLOC_LEN works out to an invalid constant for
the ARM sub instruction.  Change the code so that each is subtracted
independently to avoid the error.

Signed-off-by: Andrew Dyer <adyer@righthandtech.com>
1 file changed