silence cast-align warnings from clang on powerpc64

silence the following warning when compiling libthr with clang 8
for powerpc64 architecture:

usr/src/lib/libthr/arch/powerpc/include/pthread_md.h:82:10: error:
cast from 'uint8_t *' (aka 'unsigned char *') to 'struct tcb *'
increases required alignment from 1 to 8 [-Werror,-Wcast-align]
82: return ((struct tcb *)(_tp - TP_OFFSET));

Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: git_bdragon.rtk0.net, emaste, kib, jhibbits, luporl
Differential Revision: https://reviews.freebsd.org/D18807