RFR: 8225167: add NativeTypes.SCHAR signed char type

[Nick Gasson]

Hi,

As discussed at the end of the thread for the initial AArch64 port, this 
patch adds an explicitly signed char type SCHAR to NativeTypes. Because 
on some platforms both CHAR and UCHAR will be unsigned.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225167
Webrev: http://cr.openjdk.java.net/~ngasson/foreign/8225167/webrev.0/

Also made all the fields in NativeTypes final as suggested by Jorn.

Tested with jdk_foreign jtreg group on x86 and AArch64.


Thanks,
Nick