Add support for hermit and l4re

[josephlr]

Currently, getrandom supports almost all targets that rustc can build. The exceptions are:

• target_os = "none"
• target_os = "l4re"
• target_os = "hermit"

This PR adds support for the later two. Both implementations should just use RDRAND as:

• L4RE does not support a better way to get entropy from the OS.
• Hermit has OS bindings for getting randomness, but the currently implementation either:
  • Uses a broken implementation of RDRAND
  • Or falls back to an insecure implementation
  • So just using the existing RDRAND implementation in getrandom seems like the best security-wise.