`getcontext()` should not be exposed from `std.os.linux`

[alexrp]

The standard library only needs this function for stack unwinding purposes when not linking libc. Currently it's only implemented for x86 and x86_64. It should be moved to std.debug.

We shouldn't be in the business of exposing this function because std.os.linux is not a libc emulation layer, and implementing getcontext() according to libc semantics is more expensive than necessary for the standard library's needs (e.g. due to signal mask).

[mlugg]

Resolved by b64535e