[8u] RFR: 8227397: Add --with-extra-asflags configure option

[Severin Gehwolf]

Hi,

Could I please get a review of this 8u build change backport which adds
--with-extra-asflags to OpenJDK 8u. At Red Hat, we need to pass certain
assembler only flags for some builds. For example "-Wa,--generate-
missing-build-notes=yes", to assembly files only. As the build system
is different in 8u over 11u I've re-done the patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227397
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8227397/jdk8/01/

Testing: Built with --with-extra-asflags=-Wa,--generate-missing-build-
notes=yes on Linux x86_64, confirmed linux_x86_64.s gets assembled with
the flag and only that file.

I've omitted the windows portion of passing as flags to the hotspot
build as I've no idea how. Contributions welcome if that should get
included.

Thoughts?

Thanks,
Severin