RFR: JDK-8202384: Introduce altserver jvm variant with speculative execution disabled

[Erik Joelsson]

We need to add compilation flags for disabling speculative execution to 
our native libraries and executables. In order to allow for users not 
affected by problems with speculative execution to run a JVM at full 
speed, we need to be able to ship two JVM libraries - one that is 
compiled with speculative execution enabled, and one that is compiled 
without. Note that this applies to the build time C++ flags, not the 
compiler in the JVM itself. Luckily adding these flags to the rest of 
the native libraries did not have a significant performance impact so 
there is no need for making it optional there.

This patch defines flags for disabling speculative execution for GCC and 
Visual Studio and applies them to all binaries except libjvm when 
available in the compiler. It defines a new jvm feature 
no-speculative-cti, which is used to control whether to use the flags 
for libjvm. It also defines a new jvm variant "altserver" which is the 
same as server, but with this new feature added.

For Oracle builds, we are changing the default for linux-x64 and 
windows-x64 to build both server and altserver, giving the choice to the 
user which JVM they want to use. If others would prefer this default, we 
could make it default in configure as well.

The change in GensrcJFR.gmk fixes a newly introduced race that appears 
when building multiple jvm variants.

Bug: https://bugs.openjdk.java.net/browse/JDK-8202384

Webrev: http://cr.openjdk.java.net/~erikj/8202384/webrev.01

/Erik