Rework rtld's TLS Variant I implementation to match r326794

Rework rtld's TLS Variant I implementation to match r326794

The above commit fixed handling overaligned TLS segments in libc's
TLS Variant I implementation, but rtld provides its own implementation
for dynamically-linked executables which lacks these fixes.  Thus,
port these changes to rtld.

This was previously commited as r337978 and reverted in r338149 due to
exposing a bug the ARM rtld.  This bug was fixed in r338317 by mmel.

Submitted by:	James Clarke
Approved by:	re (kib)
Reviewed by:	kbowling
Testing by:	kbowling (powerpc64), br (riscv), kevans (armv7)
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D16510