Fix some arm issues

Summary: Missing vasm instructions and a difference between the x64 and arm
versions of emitCallArrayPrologue. They both crept in due to test infra issues.

Reviewed By: @mxw

Differential Revision: D1733394

Signature: t1:1733394:1418280328:317bb65b2e2c2264750c11ccee1e80844c1c2277