RFR: JDK-8211073 Remove -Wno-extra from Hotspot

[Magnus Ihse Bursie]

The -Wextra option to gcc enables a bunch of useful warnings.[1] Some of 
them, but not all, can be individually enabled or disabled. All other 
libraries in OpenJDK are compiled with -Wextra, but not Hotspot. 
Enabling -Wextra on Hotspot triggers a couple of warnings for zero that 
can be individually disabled.

However, -Wextra also includes some check that cannot be disabled 
individually, so to be able to add this, we must at the same time fix 
those warnings.

The warnings that cannot be disabled and which have been triggered in 
Hotspot is "enumeral and non-enumeral type in conditional expression" 
and "base class should be explicitly initialized in the copy 
constructor". The former complains about mixing enums and integers in 
the tertiary operator (x ? enum_val : int_val). If you think that gcc is 
a bit too picky here, I agree. It's not obvious per se that the added 
casts improve the code. However, this is the price we need to pay to be 
able to enable -Wextra, and *that* is something that is likely to 
improve the code.

The second warning about the copy constructor is, for what I can tell, a 
highly valid warning and the code it warned on was indeed broken. As far 
as I can tell, in a derived copy constructor you should always 
explicitly initialize the base class.

Bug: https://bugs.openjdk.java.net/browse/JDK-8211073
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8211073-remove-Wno-extra-from-hotspot/webrev.01

/Magnus

[1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html