So I ran this command:

$ ./pre-inst-env guix build bootstrap-tarballs --target=mips64el-linux-gnuabi64

but it failed due to the already known problem:

/tmp/nix-build-glibc-cross-mips64el-linux-gnuabi64-2.18.drv-0/build/libc_pic.a: could not read symbols: Archive has no index; run ranlib to add one

Then I tried this one:

$ ./pre-inst-env guix build bootstrap-tarballs -s x86_64-linux --target=mips64el-linux-gnuabi64

which returned

guix build: error: build failed: a `x86_64-linux' is required to build `/nix/store/v68c12la3xlxii4risamhmh1fic04jq4-gcc-4.7.3.drv', but I am a `i686-linux'

How long will it take for Hydra to catch up?  There is a percentage on
hydra.gnu.org (under the word “Success”).  Is it the thing I’m looking
for?

Also, how can I check whether this particular package [1] was built or
not?

[1] http://git.savannah.gnu.org/cgit/guix.git/commit/?id=ab4fab19380138df582fdde846e7524481964c1a