Remove unnecessary linker script preprocessing

The previous build process ran sed against the linker scripts twice:
once to replace <X> with the contents of sym_x.ld, a second time to
correct any paths referencing tools/.

The first can be replaced with INCLUDE commands, as was done in
pret/pokeemerald@7c57699 (in 2018).

The second has been unnecessary since 942c248 (also 2018).

With both of these removed there is no longer a need to have a build
step for them, and $(ELF) can take the sym_x.ld files as prerequisites
instead.

ld_script_modern.ld

@@ -0,0 +1,82 @@
+gNumMusicPlayers = 4;
+gMaxLines = 0;
+
+SECTIONS {
+    . = 0x2000000;
+
+    ewram (NOLOAD) :
+    ALIGN(4)
+    {
+        gHeap = .;
+
+        . = 0x1C000;
+
+        *(ewram_data);
+
+        . = 0x40000;
+    }
+
+    . = 0x3000000;
+
+    iwram (NOLOAD) :
+    ALIGN(4)
+    {
+        /* .bss starts at 0x3000000 */
+        *(.bss*);
+        /* .bss.code starts at 0x3001AA8 */
+        src/m4a.o(.bss.code);
+
+        /* COMMON starts at 0x30022A8 */
+        *(COMMON);
+        end = .;
+        __end__ = .;
+        . = 0x8000;
+    }
+
+    . = 0x8000000;
+
+    .text :
+    ALIGN(4)
+    {
+        src/rom_header.o(.text*);
+        src/rom_header_gf.o(.text.*);
+        src/crt0.o(.text);
+        src/main.o(.text);
+        *(.text*);
+    } =0
+
+    script_data :
+    ALIGN(4)
+    {
+    	*(script_data);
+    } =0
+
+    .data :
+    ALIGN(4)
+    {
+    	*(.data*);
+    } =0
+
+    .rodata :
+    ALIGN(4)
+    {
+    	*(.rodata*);
+    } =0
+
+    /* DWARF 2 sections */
+    .debug_aranges  0 : { *(.debug_aranges) }
+    .debug_pubnames 0 : { *(.debug_pubnames) }
+    .debug_info     0 : { *(.debug_info) }
+    .debug_abbrev   0 : { *(.debug_abbrev) }
+    .debug_line     0 : { *(.debug_line) }
+    .debug_frame    0 : { *(.debug_frame) }
+    .debug_str      0 : { *(.debug_str) }
+    .debug_loc      0 : { *(.debug_loc) }
+    .debug_macinfo  0 : { *(.debug_macinfo) }
+
+    /* Discard everything not specifically mentioned above. */
+    /DISCARD/ :
+    {
+        *(*);
+    }
+}