powerpc: Kernel fixes for ppc32 and powerpcspe w/ lld

Fix wrong section ordering that was causing a ".got is not contiguous with
other relro sections" lld error. This also brings ldscript.powerpc and
ldscript.powerpcspe closer to ldscript.powerpc64.

Also, remove unnecessary text relocs from the ppc32 AIM trap code.

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D22349