The get_cpuid() function in cpu-x86.c was buggy on x86-64. When building
without optimizations, the homegrown assembly code overwrote the
beginning of the function argument list on the stack. That happened to
work fine on regular x86-64, but caused crashing with the x32 ABI.

At least GCC and clang provide cpuid.h, which has the __get_cpuid()
function that can be used instead of the homegrown assembly.

The PA_REG_* constants can be removed as well, because they're not used
any more.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=103656