RFR (M): 8188764: Obsolete AssumeMP and then remove all support for non-MP builds

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8188764
Webrev: http://cr.openjdk.java.net/~dholmes/8188764/webrev/

As previously discussed in the RFC thread:

http://mail.openjdk.java.net/pipermail/hotspot-dev/2018-September/034166.html

this change obsoletes the AssumeMP flag and removes the bulk of the 
logic that is conditional on os::is_MP() as follows:

1. to gate the introduction of MP-specific features, notably memory barriers

The is_MP check is removed and we will always issue memory barriers
or pre-pend lock prefix etc.

2. to align certain patchable code sequences (method entry, call sites)

The is_MP is removed and we always align patchable locations.

3. to gate certain optimizations which are questionable on uniprocessors
(see quicken_jni_functions)

These are psuedo-memory-barriers where we manufacture a data-dependency
instead of inserting mfence/lfence (x86 example). These are treated as
#1 and is_MP is removed.

4. to gate optimizations which are desirable on uniprocessors
(mutex/monitor short circuits)

These are spin controls and so is_MP remains.

I have not updated the about-to-be-removed Oracle ARM port code.

Testing:
   - Tiers 1 -3 (mach5)

Performance run TBD.

Thanks,
David