Use --hash-style=gnu for gcc

When support for gnu hash tables were added to the gcc toolchains, the OpenJDK build system added a hack to change --hash-style=gnu to --hash-style=both unconditionally, citing compatibility concerns.

--hash-style=gnu has a very slight performance advantage over --hash-style=both (very small static footprint reductions, tiny startup cost differences). Both --hash-style=gnu and --hash-style=both have a small to potentially large performance advantage over --hash-style=sysv

We should make either --hash-style=both or --hash-style=gnu the default.

I'd prefer making --hash-style=gnu the default, and allow users to configure and build with an explicit --hash-style of their own choice (gnu, sysv, both) when building for legacy OSes which doesn't support gnu-only.