Currently, Torque's builtin pointers store a Code target underneath and
callsites generate a kArchCallCodeObject opcode. When embedded builtins
are enabled, the call thus first calls the on-heap trampoline, which
finally jumps to the target off-heap builtin code.

This will no longer be possible in jitless mode, since on-heap code must
not be executable.

As a step towards changing the way builtin pointers are called
(function pointers will hold the builtin index as a Smi, and callsites
look up the off-heap target address and jump there), this CL adds a
dedicated opcode for builtin pointer calls to the compiler pipeline.

The calling mechanism itself is unchanged, changes there will happen
in a follow-up.

Drive-by: rename 'FunctionPointer' in torque/ to 'BuiltinPointer'.

Bug: v8:7777
Change-Id: Ic999a1cd7c3172425dd4a1513ae2f50c774faddb
Reviewed-on: https://chromium-review.googlesource.com/c/1378175Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58281}