Added pce-specific substitutes for memcpy() and memmove().

They are smaller and faster because they take advantage of the pce CPU's block-copy instructions.

Also, made a small improvement to the common memmove(), so that it is similar to the pce version.

libsrc/pce/memmove.s

@@ -0,0 +1,63 @@
+;
+; This file, instead of "common/memmove.s", will be assembled for the pce
+; target.  This version is smaller and faster because it uses the HuC6280's
+; block-copy instructions.
+;
+; 2003-08-20, Ullrich von Bassewitz
+; 2015-10-23, Greg King
+;
+; void* __fastcall__ memmove (void* dest, const void* src, size_t size);
+;
+; NOTE: This function uses entry points from "pce/memcpy.s"!
+;
+
+        .export         _memmove
+
+        .import         memcpy_getparams, memcpy_increment, memcpy_transfer
+        .importzp       ptr1, ptr2, ptr3
+
+        .macpack        generic
+        .macpack        longbranch
+
+
+; ----------------------------------------------------------------------
+_memmove:
+        jsr     memcpy_getparams
+
+; Check for the copy direction.  If dest < src, we must copy downwards (start
+; at low addresses, and increase pointers); otherwise, we must copy upwards
+; (start at high addresses, and decrease pointers).
+
+        cmp     ptr1
+        txa
+        sbc     ptr1+1
+        jcc     memcpy_increment        ; Branch if dest < src
+
+; Copy decrementing; adjust the pointers to the end of the memory regions.
+
+        lda     ptr1
+        add     ptr3
+        sta     ptr1
+        lda     ptr1+1
+        adc     ptr3+1
+        sta     ptr1+1
+
+        lda     ptr1                    ; point to last byte of source
+        bne     @L1
+        dec     ptr1+1
+@L1:    dec     ptr1
+
+        lda     ptr2
+        add     ptr3
+        sta     ptr2
+        lda     ptr2+1
+        adc     ptr3+1
+        sta     ptr2+1
+
+        lda     ptr2                    ; point to last byte of target
+        bne     @L2
+        dec     ptr2+1
+@L2:    dec     ptr2
+
+        ldy     #$C3                    ; TDD
+        jmp     memcpy_transfer