Fix bugs in cpuid.h.

This commit makes two changes:

- Remove the push and pop instructions that were saving and restoring %ebx
  before and after cpuid in 32-bit pic mode. We were doing this to ensure we
  don't lose the GOT address in pic register %ebx, but this isn't necessary
  because the GOT address is kept in a virtual register.

- In 64-bit mode, preserve base register %rbx around cpuid.

This fixes PR20311 and rdar://problem/17686779.