arm: Guard ucontext _rtld_global_ro access by SHARED, not PIC macro

Due to PIE-by-default, PIC is now defined in more cases.  libc.a
does not have _rtld_global_ro, and statically linking setcontext
fails.  SHARED is the right condition to use, so that libc.a
references _dl_hwcap instead of _rtld_global_ro.

For static PIE support, the !SHARED case would still have to be made
PIC.  This patch does not achieve that.

Fixes commit 23645707
("Replace --enable-static-pie with --disable-default-pie").

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>