RFR: 8181147: JNI_GetStringPlatformChars should have a fast path for UTF-8

[Claes Redestad]

Hi,

various JNI methods in the JDK converts from java Strings to native
encoding using JNI_GetStringPlatformChars, which has long standing
optimizations for dealing with various default charsets.

However, UTF-8 is missing, which is a shame since we have optimized
utilities for converting from a String to UTF-8-encoded char* (since
this is the native encoding used by HotSpot internally).

Webrev: http://cr.openjdk.java.net/~redestad/8181147/jdk.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8181147

Allocation rate can drop significantly in microbenchmarks, e.g., -60%
in a trivial micro doing new File(path).isHidden() (along with a 30%
throughput win), while the rate of native allocation with is net neutral.

(I think the code for FAST_646_US could be removed, since Solaris 8
support was dropped in JDK 8, but that's a separate RFE...)

Thanks!

/Claes