Fix armhf build for GCC-11

[infinity0]

GCC 11 changed the default flag to put the FP option in the -march flag (arm7-a+fp) rather than a specific FPU flag. Therefore it needs to be given separately now.

See https://bugs.launchpad.net/ubuntu/+source/gcc-defaults/+bug/1939379 for some more details.

Example build failure is here: https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=armhf&ver=1.55.0%2Bdfsg1-1&stamp=1634161880&raw=0

[infinity0]

Note: the snippet I added is already part of another armv7-a case further below in the file. I'm actually not entirely clear why there are 2 cases to handle these similar situations; both add armv7-a as a flag, but are based on the slightly different conditions target.starts_with("armv7a") vs target.starts_with("armv7"). Anyway, it works to fix the Debian build failure.

[infinity0]

Working build with this patch applied: https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=armhf&ver=1.55.0%2Bdfsg1-2&stamp=1634177225&raw=0

[alexcrichton]

Personally I have no idea how these flags are managed, this crate is just the conglomeration of years of adding various flags, it's not like I understand 100% what all the flags are (or anyone else really). If this works for you then it's probably fine for others too.