lib/syscall_shim: Use global scope inline asm for execenv prologue

lib/syscall_shim: Use global scope inline asm for execenv prologue

Avoid having to use weird and unconventional function attributes by
writing the inline assembly in a global scope. This is especially
useful for ARM64 as it does not support the naked attribute on GCC.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
GitHub-Closes: #1598