cpu/internal: provide runtime detection of RISC-V extensions on Linux
Add a RISCV64 variable to cpu/internal that indicates both the presence
of RISC-V extensions and performance information about the underlying
RISC-V cores. The variable is only populated with non false values on
Linux. The detection code relies on the riscv_hwprobe syscall
introduced in Linux 6.4. The patch can detect RVV 1.0 and whether
the CPU supports fast misaligned accesses. It can only detect RVV 1.0
on a 6.5 kernel or later (without backports).
Updates #61416
Change-Id: I2d8289345c885b699afff441d417cae38f6bdc54
Reviewed-on: https://go-review.googlesource.com/c/go/+/522995 Reviewed-by: Joel Sing <joel@sing.id.au> Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: David Chase <drchase@google.com>