Please add a feature to switch to a basic implementation

[glandium]

Behind this generic request, here is my actual problem: running code that uses memchr under miri fails with the following:

error: unsupported operation: can't call foreign function `llvm.aarch64.neon.umaxp.v16i8` on OS `macos`
    --> /Users/glandium/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/aarch64/neon/mod.rs:1903:5
     |
1903 |     vpmaxq_u8_(a, b)
     |     ^^^^^^^^^^^^^^^^ can't call foreign function `llvm.aarch64.neon.umaxp.v16i8` on OS `macos`
     |
     = help: if this is a basic API commonly used on this target, please report an issue with Miri
     = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases

where the call to this function originates from memchr.

A feature that switches to use a dumb implementation could be useful for cases like this.