[compiler-rt] Change all asm directives to __asm__ in cpu_model.

Summary:
Aiming to resolve failure in sanitizer bot by changning all asm directives to __asm__.
Also, consistency.

Reviewers: llvm-commits, echristo, vitalybuka

Subscribers: mehdi_amini

Differential Revision: https://reviews.llvm.org/D22446

llvm-svn: 275740