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.

(cherry picked from commit 48326cfb1ef4b455f81fbbece7a17dab17f7c932)

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>

(cherry picked from commit 99484c965c93df5367ecb3bc29165544baf1da76)
Bump win32 version number in release notes

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

(cherry picked from commit 044f23388c0c316fbc470c93860797a30b3130bd)