disable LTO in RISC-V to speed up build

[mmoll]

Closes #1011

[yosifkit]

After doing some emulated, local tests, I think this seems reasonable to try again. It'll be a less optimized python, but that's better than no riscv64 python image. (though users could just install the alpine-provided python3 package in an image FROM alpine, but then they are limited to a specific version)

None of these builds are as slow as I was getting in #931 and it's on the same machine. 😕 🤷

$ # --with-lto
$ for ver in 3.10 3.11 3.12; do time docker build --no-cache --platform linux/riscv64 ./$ver/alpine3.20/; done
real    19m13.374s

real    30m32.618s

real    33m12.560s

$ # without lto
$ for ver in 3.10 3.11 3.12; do time docker build --no-cache --platform linux/riscv64 ./$ver/alpine3.20/; done
real    8m49.526s

real    9m41.161s

real    9m22.683s

---

Requested change: also remove this part of generate-stackbrew-library.sh to stop removing the riscv64 from the architecture lists.

[tianon]

Suggested change

	# <3.10 does not have -fno-semantic-interposition enabled and --with-lto does nothing for performance
	# <3.10 does not have -fno-semantic-interposition enabled and --with-lto does nothing for performance
	# skip LTO on alpine on riscv64: https://github.com/docker-library/python/pull/935, https://github.com/docker-library/python/pull/1038

[mmoll]

thanks for the comments, should all be done