The Mach-O executable format, used on macOS/Darwin, stores function
names in mangled form, so we need to mangle it ourselves during
function pointer lookup.

Bug b/115344057

Change-Id: I8de5756c52b5af666826134fca8274b4467fa85a
Reviewed-on: https://swiftshader-review.googlesource.com/c/22188Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>