[crypto] Harden RSA modexp subroutine. #3598

Job	Run time
Lint (quick)	3m 26s
Lint (slow)	15m 51s
Build documentation	5m 32s
CW305's Bitstream	21m 53s
Run OTBN crypto tests	21m 24s
Airgapped build	11m 52s
Run OTBN smoke Test	2m 37s
Verible lint	1m 45s
Verilated Earl Grey	1h 24m 12s
Verilated English Breakfast	8m 19s
Build Docker Containers	2m 39s
Build and test software	17m 29s
Earl Grey for CW310 / Build bitstream	1m 52s
Earl Grey for CW310 Hyperdebug / Build bitstream	1m 54s
Earl Grey for CW340 / Build bitstream	1m 55s
CW310 ROM Tests / FPGA test	41m 49s
CW310 ROM_EXT Tests / FPGA test	7m 10s
CW310 Test ROM Tests / FPGA test	5m 48s
CW310 SiVal Tests / FPGA test	25m 14s
CW310 SiVal ROM_EXT Tests / FPGA test	36m 40s
CW310 Manufacturing Tests / FPGA test	32m 13s
Cache bitstreams to GCP	0s
CW340 ROM Tests / FPGA test	59s
CW340 ROM_EXT Tests / FPGA test	7m 3s
CW340 Manufacturing Tests / FPGA test	7m 53s
CW340 SiVal Tests / FPGA test	18m 23s
CW340 SiVal ROM_EXT Tests / FPGA test	4m 28s
CW340 Test ROM Tests / FPGA test	3m 58s
Verify FPGA jobs	25s
	6h 34m 43s

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