LoadCurrentCharacterImpl is implemented once in each of the eight
regexp-macro-assembler-<arch>.cc files. Aside from small differences
in comment wording, those eight implementations are identical. The
architecture-specific code for LoadCurrentCharacter is all in
LoadCurrentCharacterUnchecked.

This patch hoists the definition of LoadCurrentCharacterImpl into
NativeRegExpMacroAssembler and turns LoadCurrentCharacterUnchecked
into a virtual function.

Note: The arm64 version of LoadCurrentCharacterImpl contained the
following six-year-old comment, which I don't think is worth
preserving:

// TODO(pielan): Make sure long strings are caught before this, and
// not just asserted in debug mode.

R=jgruber@chromium.org

Bug: v8:10406
Change-Id: Ic81283ad3b618d6b06f4206fb77d30de617dccb7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2140003
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67260}