Add a perf-cross build setting.
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)

hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay