MFC 354289,354441,355095: Use lld on armv6.

354289:
armv6: Switch to LLD by default

This could just be ${__TT} == "arm", except armv5 isn't slated for death until
EOY.

arm tinderbox builds. Let's see what else shakes out.

354441:
arch.7: armv6 uses lld by default as of r354289

355095:
remove armv6 LLVM workaround introduced in r341812

r341812 enabled only arm target support in LLVM on arm and armv6,
because ld.bfd 2.17.50 lacked support for range extensions required for
linking such large binaries/libraries. r341812 indicated that the
workaround should be removed once the userland can be linked by lld.

r354289 switched armv6 to use lld by default, so remove the workaround
on armv6. The workaround remains in place for arm (v5), and will
presumably be removed when arm is retired.