Update stdarch submodule (to before it switched to const generics)

This also includes a cherry-pick of
https://github.com/rust-lang/stdarch/commit/ec1461905b421cf0c56adeebb49bbf55bb33fd17
and https://github.com/rust-lang/stdarch/pull/1108 to fix a build
failure.

It also adds a re-export of various macros to the crate root of libstd -
previously they would show up automatically because std_detect was defined
in the same crate.

1 files changed, 0 insertions, 0 deletions

diff --git a/library/stdarch b/library/stdarch
-Subproject 9c732a56f67f54d12a0b4fd99993154906c95ea
+Subproject 19f5459dd0f89e466b7bcaa0f69ecca90f21a4d