RFR: 8257985: count_trailing_zeros doesn't handle 64-bit values on 32-bit JVM [v8]

[Claes Redestad]

> Re-implement count_trailing_zeros as a template function similar to count_leading_zeros, adding support for 8 and 16 bit types as well as for 64-bit types on 32-bit builds, which is currently not supported. This prohibits implementing the log2i_exact proposed by #1663 using count_trailing_zeros, as suggested during review.
> 
> Compared to count_leading_zeros the implementation can be slightly simpler since subword specializations can simply use the internal 32-bit variant.
> 
> Windows doesn't define _BitScanForward64 on 32-bit targets, so the 32-bit variant for handling 64-bit values becomes more convoluted.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Windows impl. cleanup

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1747/files
  - new: https://git.openjdk.java.net/jdk/pull/1747/files/a7829d5e..f65a7f5e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1747&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1747&range=06-07

  Stats: 5 lines in 1 file changed: 1 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1747.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1747/head:pull/1747

PR: https://git.openjdk.java.net/jdk/pull/1747