arch: x86: Rename MSR-handling functions to conform to convention

Rename _MsrRead() and _MsrWrite() to _x86_msr_read() and
_x86_msr_write() respectively.

Given that these functions are essentially implemented in assembly.
make them static inline.  They can be inlined by the compiler quite
well, most of the time incurring in space savings due to better
handling of the cobbled registers.

Also simplifies the inline assembly, using constraints instead of
moving registers ourselves.  Should shave off a few bytes from code
using these functions.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>