math: remove assembly stubs

[mundaym]

I could be wrong, but I think now that we have mid-stack inlining it should be possible to move the math package from this pattern:

// F does blah.
func F() // implemented in asm, might just jump straight to f

func f() {
    // generic implementation in Go
}

To something more like this pattern:

// F does blah.
func F() {
    f() // might call asm, might just call fGeneric
}

func fGeneric() {
    // generic implementation in Go
}

This would have the side benefit of allowing architectures that use the generic implementation to inline it if it is cheap enough. It might also make stdlib easier to build with other compilers (#31330).

[gopherbot]

Change https://golang.org/cl/173398 mentions this issue: math: consolidate assembly stub implementations