x86_64: Remove unused padding from tcbhead_t

x86_64: Remove unused padding from tcbhead_t

This padding is difficult to use for preserving the internal
GLIBC_PRIVATE ABI.  The comment is misleading.  Current Address
Sanitizer uses heuristics to determine struct pthread size.
It does not depend on its precise layout.  It merely scans for
pointers allocated using malloc.

Due to the removal of the padding, the assert for its start
is no longer required.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>