arm64: Move curthread setup earlier

In 469cfa3c30ee cperciva added TSLOG profiling to link_elf_ireloc. This
requires curthread to be read when the kernel linker is invoked, but it
hadn't yet been initialized. On amd64 this was harmless since [gs:0] was
readable; but on arm64 this broke since [x18] was not readable.

Move the curthread (and associated PCPU) setup earlier on arm64 in order
to allow TSLOG to work there.

Fixes: 469cfa3c30ee ("tslog: Annotate some early boot functions")
Differential Revision: https://reviews.freebsd.org/D44317