pmcstat: fix PPC kernel symbol resolution

PowerPC kernel is of DYN type and it has a base address where it is
initially loaded, before being relocated. As the start address passed to
pmcstat_image_link() is where the kernel was relocated to, but the symbols
always use the original base address, we need to subtract it to get the
correct offset.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D26114