ghc.mk: one module name per line

Signed-off-by: Austin Seipp <austin@well-typed.com>
Rejigger flushExec implementation (#8562, #8561)

Instead, just don't do anything on x86/amd64, and on !x86, use either A)
__clear_cache from libgcc, or B) sys_icache_invalidate for OS X (and
iOS.)

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