cmd/compile: remove ARMv5 special case in register allocator

The register allocator has a special case that doesn't allocate
LR on ARMv5. This was necessary when softfloat expansion was done
by the assembler. Now softfloat calls are inserted by SSA, so it
works as normal. Remove this special case.

Change-Id: I5502f07597f4d4b675dc16b6b0d7cb47e1e8974b
Reviewed-on: https://go-review.googlesource.com/c/go/+/301792


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>