cross-compilation broken for 32-bit builds

[Kiskae]

Describe the bug
when cross-compiling for 32-bit linux, the libc::sysconf call returns a 32-bit integer which is incompatible with the i64 it wants to be stored in.

PR #430 made this change but it also removed the cast that made the previous version work on all platforms.

To Reproduce

Cross-compile using the i686-unknown-linux-musl target.

Error: --> /cargo/registry/src/github.com-1ecc6299db9ec823/prometheus-0.13.1/src/process_collector.rs:199:13
|
197 | static ref PAGESIZE: i64 = {
| --- expected i64 because of return type
198 | unsafe {
199 | libc::sysconf(libc::_SC_PAGESIZE)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected i64, found i32
|
help: you can convert an i32 to an i64
|
199 | libc::sysconf(libc::_SC_PAGESIZE).into()
| +++++++

For more information about this error, try rustc --explain E0308.
error: could not compile prometheus due to 2 previous errors

Expected behavior
There is no reason why it shouldn't be able to compile on 32-bit systems.

System information

• CPU architecture:
  • x86_64
• Distribution and kernel version:
  • N/A
• SELinux on?:
  • N/A
• Any other system details we should know?:
  • Rust toolchain: stable-x86_64-unknown-linux-gnu unchanged - rustc 1.61.0 (fe5b13d68 2022-05-18)

Additional context
Ran into this problem while building multi-arch docker images. It might be an idea to add cross-compilation to CI to catch problems like this in the future