x86/hyperv: Use any general-purpose register when saving %cr2 and %cr8

hv_hvcrash_ctxt_save() in arch/x86/hyperv/hv_crash.c currently saves %cr2
and %cr8 using %eax ("=a"). This unnecessarily forces a specific register.
Update the inline assembly to use a general-purpose register ("=r") for
both %cr2 and %cr8. This makes the code more flexible for the compiler
while producing the same saved context contents.

No functional changes.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>