x86 tsc: mark %eax as earlyclobber in tscp_get_timecount_low().

i386 codegen insists on preloading tc_priv into register on i386, and
this register cannot be %eax because RDTSCP instruction clobbers it
before it is used.

Reported and tested by: dim
MFC after: 6 days
Sponsored by: The FreeBSD Foundation