This patch renames PDB/calling-conventions.test to calling-conventions-x86.test.
Also restrict it to run only for target-x86*.
This patch also adds a arm specific test PDB/calling-conventions-arm.test which
tests that x86 specifc calling convention decorators are ignored by Arm compiler.
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D128678
