Adjust r308689 to make rtld compilable with either in-tree or
(hopefully) stock gcc 4.2.1 on i386 and other arches.

In particular:

• Do not use %ebx in the asm constraints on i386, since rtld is compiled with -fPIC and gcc cannot handle GOT-base register reload (clang and newer gcc can).
• Avoid direct use of [static N] construct in the function declaration/definion. In-tree gcc was patched to support this, but stock 4.2.1 cannot handle the feature.

Requested by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week