Support compiling against Musl

[simonjbeaumont]

Motivation

When building for Linux, we would like to be able to build this package on top of Musl, as well as the usual Glibc.

Modifications

Replace imports of Glibc with conditional imports using the following pattern:

- import Glibc
+ #if canImport(Glibc)
+ import Glibc
+ #elseif canImport(Musl)
+ import Musl
+ #else
+ #error("Unsupported runtime")
+ #endif

Result

Can compile against Musl.

[ktoso]

@swift-server-bot test this please

[ktoso]

Thank you!