Revert "rtld: fix allocate_module_tls() variant I fallback to static allocation"

This was applying a NetBSD fix to FreeBSD. However, the original code
was correct for FreeBSD. NetBSD's obj->tlsoffset is relative to the end
of the TCB, not the TCB itself, whilst ours is relative to the TCB[1]
itself. For example, our allocate_tls uses (char *)tcb + obj->tlsoffset
for the memcpy and memset calls.

Without this reverted, for dynamically loaded shared objects, Initial
Exec accesses to TLS variables on variant I architectures (non-x86) use
the correct address, whilst General Dynamic and dlsym(3) use the
incorrect address (TLS_TCB_SIZE past the start). Note that, on arm64,
LLVM only supports TLSDESC (including LLD) and TLSDESC will use the
static resolver if the variable ends up allocated to the static TLS
block, even in the presence of dlopen(3), so only dlsym(3) shows the
discrepancy there.

Whilst here, add a comment to explain this difference to try and avoid
the same mistake being made in future.

[1] In the case of variant II, it's the amount to subtract, so still

positive

This reverts commit e9a38ed2fa61fd264a80f24ceb35f39b0ac6463d.

Reviewed by: kib (prior version)
Fixes: e9a38ed2fa61 ("rtld: fix allocate_module_tls() variant I fallback to static allocation")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50565