RFR: JDK-8239794 Move -Os from JVM feature 'minimal' to new feature 'opt-size'

[Magnus Ihse Bursie]

Prior to JDK-8239450, it was possible to explicitly set the JVM feature 
'minimal', even when building variant server. The reason for doing this 
was to get the code in JvmFeatures.gmk that sets JVM_OPTIMIZATION := 
SIZE (except for a handful of files).

Since this functionality is important in itself (and it has been 
requested by users in the community), it should be controlled by a 
separate feature, instead of just as a part of the 'minimal' feature, 
which signals that the minimal JVM variant is being built.

However, when looking into it, I noticed several aspects of the old code 
that I wanted to address.

First of all, the old code had the size optimization silently ignored, 
if also 'link-time-opt' was enabled. This does not make any sense, since 
it is completely possible to do both. This code was written in the new 
Hotspot build system as a way to mimick the old Hotspot build system. 
I'm guessing that this was initially introduced as a way to enable the 
size optimization when building minimal, except for arm-32 systems, 
which were the only system to have link-time-opt enabled.

After some head scratching, I implemented the logic in the following way:
  * If opt-size is enabled, the JVM_OPTIMIZATION := SIZE is always 
performed, regardless of any other features.
  * And contrary, link-time-opt can be enabled/disabled independent on 
opt-size
  * When building minimal on arm-32, opt-size is not enabled by default, 
but link-time-opt is (as it was before)
  * When building minimal on any other platform, opt-size is enabled by 
default, but link-time-opt is not (as it was before)
* When building any other variant, opt-size is disabled by default

This makes standard builds of minimal with no extra features 
enabled/disabled behave the same as before, while giving the maximum of 
flexibility to change this. But I'd like to get input from someone in 
the Hotspot team that this is a sound approach.

Also, this has apparently never been tested by any other compiler than 
gcc and/or clang, since we explicitly add the compiler flag 
-fno-optimize-sibling-calls. I wrapped this in a compiler check.

Note that this patch is based on JDK-8239789 (Follow-up on JVM feature 
rewrite).

Bug: https://bugs.openjdk.java.net/browse/JDK-8239794
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8239794-create-opt-size-jvm-feature/webrev.01

/Magnus