RFR: 8240259: Disable -Wshift-negative-value warnings

[Kim Barrett]

Please review this change which disables warnings for left shift of a
negative value when compiling HotSpot with gcc or clang.  This warning
isn't helpful, given that all compilers seem to do the obvious thing,
and that obvious thing will soon-ish (C++20) be the specified thing.
See the CR for more details.

CR:
https://bugs.openjdk.java.net/browse/JDK-8240259

Webrev:
https://cr.openjdk.java.net/~kbarrett/8240259/open.00/

Testing:
mach5 tier1.  Also tier1-5 with C++14 enabled.

Note that there are already occurrences of left shift of negative
values in our code, with no apparent consequences, even when testing
with C++11/14 enabled.