[InstrProf] Fix macOS profile tests after D156569

In https://reviews.llvm.org/D156569 we changed the format of the IRPGO
counter names which broke some macOS tests because the `__profc_`
variable names changed. Use `{{_?}}` to allow mangled names to be
prefixed with `_` to pass tests.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D157321