The compiler is invoked using the upstream compiler name.  TODO: use
the hexagon-unknown-none-elf-clang target instead.

The starting location is adjusted to avoid this error:

    hexagon-unknown-linux-musl-clang -o minivm -T hexagon.lds minivm.o -nostdlib
    ld.lld: error: section .vm_data at 0xFFFF8000 of size 0x8488 exceeds available address space
    hexagon-unknown-linux-musl-clang: error: hexagon-ld command failed with exit code 1 (use -v to see invocation)

Signed-off-by: Brian Cain <bcain@quicinc.com>

Before, SSR bits 19-21 were 'TNUM' but now this info is in the
`htid` register aka s8.

Actually to s20 as the open source toolchain doesn't have this alias
implemented yet.