[Bug inline-asm/87733] local register variable not honored with earlyclobber

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87733

--- Comment #28 from Rich Felker <bugdal at aerifal dot cx> ---
And it looks like I actually hit this exact bug back in 2012 but misattributed
it:

https://git.musl-libc.org/cgit/musl/commit/?id=4221f154ff29ab0d6be1e7beaa5ea2d1731bc58e

I assumed things went haywire from using two separate "r" constraints, rather
than "r" and "0", to bind the same register, but it seems the real problem was
that the "=&r"(r2) was not binding at all, and the "0"(r2) served to fix that.