Add a `Symbolizer::GetEnvP()` method that allows symbolizer implementations to customise the environment of the symbolizer binary.

Summary:
This change introduces the `Symbolizer::GetEnvP()` method that returns a
pointer to environment array used for spawning the symbolizer process.
The motivation is to allow implementations to customise the environment
if required.  The default implementation just returns
`__sanitizer::GetEnviron()` which (provided it's implemented) should
preserve the existing behaviours of the various implementations.

This change has been plumbed through the `internal_spawn(...)` and
`StartSubprocess(...)` process spawning implementations.

For the `StartSubprocess()` implementation we need to call `execve()`
rather than `execv()` to pass the environment. However, it appears that
`internal_execve(...)` exists in sanitizer_common so this patch use that
which seems like a nice clean up.

Support in the Windows implementation of
`SymbolizerProcess:StartSymbolizerSubprocess()` has not been added
because the Windows sanitizer runtime doesn't implement `GetEnviron()`.

rdar://problem/58789439

Reviewers: kubamracek, yln, dvyukov, vitalybuka, eugenis, phosek, aizatsky, rnk

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76666