RFR: 8368841: X25519 implementation differs from the specification in RFC 7748 [v4]

[Ben Perez]

> Our implementation of X25519 does not zero the MSB of the `u` coordinate as required by RFC 7748. Currently the test `jdk/sun/security/ec/xec/TestXDH.java` zeroes the bit manually while parsing test vectors, which is likely why this went uncaught.

Ben Perez has updated the pull request incrementally with one additional commit since the last revision:

  moved hexStringToBigInteger from Convert.java to TestXDH.java, no longer clears MSB

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29750/files
  - new: https://git.openjdk.org/jdk/pull/29750/files/0dd0c9d3..e00bd0fe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29750&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29750&range=02-03

  Stats: 37 lines in 2 files changed: 16 ins; 19 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29750.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29750/head:pull/29750

PR: https://git.openjdk.org/jdk/pull/29750