RFR: 8174831: Reduce number of Charset classes loaded on bootstrap

[Claes Redestad]

Hi,

this patch addresses a few tiny startup regressions incurred mostly by
the fact that Compact Strings eagerly loads more Charsets early.

java.nio.charset.StandardCharsets is dropped from the early bootstrap,
load fastpaths are added for the 3 eagerly used Charsets (which avoids
some early reflection use), plus a couple of other nits:

Webrev: http://cr.openjdk.java.net/~redestad/8174831/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8174831

Testing: jdk-tier1 tests

Thanks!

/Claes