docs: do not document CT_KERNEL_ARCH, it's going away...
Since we merged the 32/64-bit archs, and we explicitly used
the Linux kernel arch name, we can get rid of CT_KERNEL_ARCH.
1 reporter_name="Yann E. MORIN"
2 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"
3 reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."