[syscalls] Update mod exp compute cost (#2807) #519

Job	Run time
cargo-test-sbf (token/program)	7m 34s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	14m 23s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	9m 33s
cargo-test-sbf (token-upgrade/program)	7m 42s
cargo-test-sbf (feature-proposal/program)	7m 10s
cargo-test-sbf (governance/addin-mock/program, governance/program)	11m 34s
check	2m 48s
cargo-test-sbf (memo/program)	7m 35s
cargo-test-sbf (name-service/program)	10m 17s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	15m 45s
cargo-test-sbf (stake-pool/program)	11m 45s
test (single-pool/cli, single-pool/program)	16m 4s
cargo-test-sbf (single-pool/program)	8m 25s
test (token-upgrade/cli, token-upgrade/program)	14m 20s
	2h 24m 55s

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