Fix bn_assert_fits_in_bytes for big-endian

[justsmth]

Issues:

N/A

Description of changes:

• A recently merged upstream commit broke big-endian.
• This change fixes bn_assert_fits_in_bytes for big-endian and adds new testing for the function.

Call-outs:

N/A

Testing:

PPC32:

❯ ppc-qemu.sh crypto/crypto_test --gtest_filter="BNAssertTest.*" 2>/dev/null
Note: Google Test filter = BNAssertTest.*
[==========] Running 4 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 4 tests from BNAssertTest
[ RUN      ] BNAssertTest.assert_fits_in_bytes_large
[       OK ] BNAssertTest.assert_fits_in_bytes_large (13019 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_small
[       OK ] BNAssertTest.assert_fits_in_bytes_small (6170 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_zero
[       OK ] BNAssertTest.assert_fits_in_bytes_zero (1 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_boundary
[       OK ] BNAssertTest.assert_fits_in_bytes_boundary (16484 ms)
[----------] 4 tests from BNAssertTest (35682 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test suite ran. (35693 ms total)
[  PASSED  ] 4 tests.

PPC64:

❯ ppc64-qemu.sh crypto/crypto_test --gtest_filter="BNAssertTest.*" 2>/dev/null
Note: Google Test filter = BNAssertTest.*
[==========] Running 4 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 4 tests from BNAssertTest
[ RUN      ] BNAssertTest.assert_fits_in_bytes_large
[       OK ] BNAssertTest.assert_fits_in_bytes_large (16466 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_small
[       OK ] BNAssertTest.assert_fits_in_bytes_small (6157 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_zero
[       OK ] BNAssertTest.assert_fits_in_bytes_zero (0 ms)
[ RUN      ] BNAssertTest.assert_fits_in_bytes_boundary
[       OK ] BNAssertTest.assert_fits_in_bytes_boundary (13748 ms)
[----------] 4 tests from BNAssertTest (36375 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test suite ran. (36382 ms total)
[  PASSED  ] 4 tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.