ARM64: build librsvg without the -Zbuild-std feature flag

[kleisauke]

This feature flag is no longer necessary after PR rust-lang/compiler-builtins#444.

The only downside I can think of is that the standard library is now built without panic_immediate_abort.

[lovell]

Thanks Kleis, this definitely simplifies things and speeds up the ARM64 builds by about 10%.

The final binary size is a little bigger, as expected, perhaps ~4%. There's a big difference in the size of the intermediate librsvg static library, which highlights how much extra code the #[derive(Debug)] trait can result in.

Before:

-rw-r--r--    1 root     root       8942772 Dec 20 21:53 librsvg-2.a
-rwxr-xr-x    1 root     root      17326880 Dec 20 21:56 libvips-cpp.so.42.14.1

After:

-rw-r--r--    1 root     root      12015254 Dec 21 10:21 librsvg-2.a
-rwxr-xr-x    1 root     root      17953656 Dec 21 10:24 libvips-cpp.so.42.14.1