sim: Add a mechanism to translate ABIs to call host funcs from a TC.

sim: Add a mechanism to translate ABIs to call host funcs from a TC.

The guest ABI is specified as a template parameter. This makes it
possible for host simcall handlers to be called through different ABIs
which might be from different ISAs, or might be from different contexts
within the same ISA (32 vs 64 bit, syscall vs. function vs.
pseudo instrunction vs. semihosting call).

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I66a0f558e9c1f70a142b69b0dd95bd71e41d898b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23171
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>