fix arm __tlsdesc_dynamic when built as thumb code without __ARM_ARCH>=5

fix arm __tlsdesc_dynamic when built as thumb code without __ARM_ARCH>=5

we don't actually support building asm source files as thumb1, but
it's possible that the condition __ARM_ARCH>=5 would be false on old
compilers that did not define __ARM_ARCH at all. avoiding that would
require enumerating all of the possible __ARM_ARCH_*__ macros for
testing.

as noted in commit 05870abeaac0588fb9115cfd11f96880a0af2108, mov lr,pc
is not valid for saving a return address when in thumb mode. since
this code is a hot path (dynamic TLS access), don't do the out-of-line
bl->bx chaining to save the return value; instead, use the fact that
this file is preprocessed asm to add the missing thumb bit with an add
in place of the mov.

the change here does not affect builds for ISA levels new enough to
have a thread pointer read instruction, or for armv5 and later as long
as the compiler properly defines __ARM_ARCH, or for any build as arm
(not thumb) code. it's likely that it makes no difference whatsoever
to any present-day practical build environments, but nonetheless now
it's safe.

as an alternative, we could just assume __thumb__ implies availability
of blx since we don't support building asm source files as thumb1. I
didn't do that in order to avoid having a wrong assumption here if
that ever changes.