JDK-8200310 cleaned up java.nio.charset.StandardCharsets to not use
Charset.forName, but also removed the optimization to have
unconditionally loaded charsets set up static INSTANCE constants, which
means that many systems the java.nio.charset.StandardCharsets will be
initialized in System.initPhase1.
This should be avoided since at this point the VM is just booting up and
any code executed delays initialization of JIT etc.
This patch makes all the constants in j.n.c.StandardCharsets use new
INSTANCE fields of each respective Charset and adds some comments as to
why they exist.
I also think we should avoid depending on j.n.c.StandardCharsets within
java.base as to not accidentally execute more code in initPhase1 than
absolutely necessary, so I've changed uses in core packages (java.lang,
java.util, java.zip...) to use the internal alternative.