RFR: 8317720: RISC-V: Implement Adler32 intrinsic [v7]

[ArsenyBochkarev]

> Hello everyone! Please review this ~non-vectorized~ implementation of `_updateBytesAdler32` intrinsic. Reference implementation for AArch64 can be found [here](https://github.com/openjdk/jdk9/blob/master/hotspot/src/cpu/aarch64/vm/stubGenerator_aarch64.cpp#L3281).
> 
> ### Correctness checks
> 
> Test `test/hotspot/jtreg/compiler/intrinsics/zip/TestAdler32.java` is ok. All tier1 also passed.
> 
> ### Performance results on T-Head board
> 
> Enabled intrinsic:
> 
> | Benchmark                          |    (count) |  Mode |  Cnt  |   Score  |  Error |  Units |
> | ------------------------------------- | ----------- | ------ | --------- | ------ | --------- | ---------- |
> | Adler32.TestAdler32.testAdler32Update |      64 | thrpt  | 25 | 5522.693 | 23.387 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |     128 | thrpt |  25 | 3430.761 |  9.210 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |     256 | thrpt |  25 | 1962.888 |  5.323 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |     512 | thrpt  | 25 | 1050.938 |  0.144 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |    1024 | thrpt  | 25 |  549.227 |  0.375 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |    2048 | thrpt  | 25 |  280.829 |  0.170 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |    5012 | thrpt  | 25 |  116.333 |  0.057 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |    8192 | thrpt  | 25  |  71.392 |  0.060 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |   16384 | thrpt |  25  |  35.784 |  0.019 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update |   32768 | thrpt |  25  |  17.924 |  0.010 | ops/ms |
> | Adler32.TestAdler32.testAdler32Update  |  65536 | thrpt |  25  |   8.940 |  0.003 | ops/ms |
> 
> Disabled intrinsic:
> 
> | Benchmark                          |    (count) |  Mode |  Cnt  |   Score  |  Error |  Units |
> | ------------------------------------- | ----------- | ------ | --------- | ------ | --------- | ---------- |
> |Adler32.TestAdler32.testAdler32Update|64|thrpt|25|655.633|5.845|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|128|thrpt|25|587.418|10.062|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|256|thrpt|25|546.675|11.598|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|512|thrpt|25|432.328|11.517|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|1024|thrpt|25|311.771|4.238|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|2048|thrpt|25|202.648|2.486|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|5012|thrpt|25|100.246|1.119|ops/ms|
> |Adler32.TestAdler32.testAdler32Update|8192|t...

ArsenyBochkarev has updated the pull request incrementally with eight additional commits since the last revision:

 - Prettify L_nmax loop
 - Add comments in functions
 - Add explanation comment for L_nmax_loop
 - Fix L_nmax_loop for big lengths
 - Fix L_by16 loop step
 - Prettify intrinsic
 - Use LMUL=4 for most of the calculations
 - Use LMUL to load multiple data in one step

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18382/files
  - new: https://git.openjdk.org/jdk/pull/18382/files/3cf649c9..be7d2551

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18382&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18382&range=05-06

  Stats: 124 lines in 1 file changed: 56 ins; 15 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/18382.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18382/head:pull/18382

PR: https://git.openjdk.org/jdk/pull/18382