Explicitly add .syntax unified when building for iOS

This is the default when building with the clang built-in assembler,
but not if using the external assembler - thus always specify it,
for clarity.

Also use the three-operand for of a sub instruction in BS_NZC_CHECK.
The same is already done in the gnu version of the macro.

This fixes building most of the arm assembly with Apple's external
assembler. While this isn't a necessary goal in itself, there's no
harm in doing this either.
Merge pull request #1257 from mstorsjo/fixup-apple-assembly

Explicitly add .syntax unified when building for iOS