mk/kernel.mk: aarch64: Use full path to target JSON spec

In that JSON spec file, we now use target-features to specify the use of
the tpidr_el1 instruction for kernel side TLS accesses. This means we
can step aside from the unfortunate LLVM requirement to use the kernel
code model in order for tpidr_el1 instructions to be emitted.

In addition, we suppress generation of NEON instructions for the kernel.
1 job for aarch64-rebase in 3 minutes and 53 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
failed #31417
img

00:03:53

 
Name Stage Failure
failed
img Test
	build/livedisk.iso.gz \
build/harddrive-efi.bin.gz \
build/livedisk-efi.iso.gz
make[1]: Entering directory '/builds/microcolonel/redox'
make[1]: *** No rule to make target 'bootloader/aarch64/**', needed by 'build/bootloader'. Stop.
make[1]: Leaving directory '/builds/microcolonel/redox'
make: *** [Makefile:69: ci-img] Error 2
Cleaning up file based variables
ERROR: Job failed: exit code 1