RFR: 8293017: Improve hash calculation parallelism in jmod/jlink

[Aleksey Shipilev]

`jmod`/`jlink` are executed during build time to produce the `jmod` and the base modules image. On slow hardware (Raspberry Pi -class, for example) and/or slow VMs (debug, interpreter-only, for example) this takes a while. Profiling shows the considerable time is spent on hashing modules either for writing out the ModuleHash attribute or for verifying the hashes are good.

Those paths can be parallelized to make them quite faster.

The major contributors to module hashing are `java.base`, `jdk.desktop` and `jdk.localedata`, so we have a significant opportunity for parallelism here. 

Motivational improvements on `make clean-images images`:


# x86_64 Server, release

# Baseline
real	0m11.895s
user	1m4.526s
sys	0m10.715s

# Patched
real	0m10.701s   ; <--- 1.1x improvement
user	1m5.097s
sys	0m11.260s

# x86_64 Zero, release

# Baseline
real	5m20.335s
user	7m7.791s
sys	0m7.258s

# Patched
real	2m23.551s  ; <--- 2.23x improvement
user	7m14.442s
sys	0m7.856s


Additional testing: 
  - [x] Linux x86_64 fastdebug, `java/util/jar`
  - [x] Linux x86_64 fastdebug, `tools/jmod`
  - [x] Linux x86_64 fastdebug, `tools/jlink`
  - [x] Linux x86_64 fastdebug `tier1`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/10060/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293017
  Stats: 60 lines in 2 files changed: 40 ins; 10 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10060.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10060/head:pull/10060

PR: https://git.openjdk.org/jdk/pull/10060