Fix iOS build (fallout from 28b031c506)

As Luke Iannini reported, the Clang iOS cross compiler apparently
doesn't support __thread for some bizarre reason, so unfortunately they
too must fall back to pthread_{get,set}specific.

Signed-off-by: Austin Seipp <austin@well-typed.com>
Add a perf-cross build setting.

This is for building performance-optimized cross compilers (e.g. the iOS
target.)

Signed-off-by: Austin Seipp <austin@well-typed.com>
Bump win32 version number in release notes

Signed-off-by: Austin Seipp <austin@well-typed.com>