[NES] Move aligned buffers to .aligned_buffers at 0x200

The NES typically has noinit buffers of high alignment; given its
paucity of RAM, this risks wasting a ton of space. Typically these
buffers are placed immediately after the hard stack at 0x200, so this
change change creates an `.aligned_buffers` section there. OAM_BUF,
VRAM_BUF, and PAL_BUF in the SDK are placed there, with alignments 256,
128, and 32, respectively. The linker already sorts by decreasing
alignment, so no space is then ever wasted for these buffers.

Fixes #219
Fixes #229

mos-platform/nes/nes.ld

@@ -37,6 +37,9 @@ SECTIONS {
        *(.nmi_end)
   } >c_readonly
   INCLUDE rodata.ld
+  /* A naturally page-aligned place (0x200) to place noinit buffers with high
+   * alignment requirement (e.g. OAM) */
+  .aligned (NOLOAD) : { *(.aligned_buffers .aligned_buffers.*) } >ram
   INCLUDE data.ld
   INCLUDE zp.ld
   INCLUDE bss.ld

mos-platform/nes/nesdoug/vram_buffer.s

@@ -3,7 +3,7 @@
 
 .zeropage VRAM_INDEX
 
-.section .noinit.vram_buf,"aw",@nobits
+.section .aligned.vram_buf,"aw",@nobits
 .weak VRAM_BUF
 .balign 128
 VRAM_BUF:

mos-platform/nes/neslib/oam_update.c

@@ -4,7 +4,7 @@
 // information.
 
 __attribute__((section(".zp.sprid"))) unsigned SPRID;
-__attribute__((weak, aligned(256), section(".noinit.oam_buf"))) char OAM_BUF[256];
+__attribute__((weak, aligned(256), section(".aligned.oam_buf"))) char OAM_BUF[256];
 
 void oam_set(char index) { SPRID = index & 0xfc; }
 

mos-platform/nes/neslib/pal_update.c

@@ -4,7 +4,7 @@
 // information.
 
 __attribute__((section(".zp.pal_update"))) volatile char PAL_UPDATE;
-__attribute__((weak, aligned(32), section(".noinit.pal_buf"))) volatile char PAL_BUF[32];
+__attribute__((weak, aligned(32), section(".aligned.pal_buf"))) volatile char PAL_BUF[32];
 
 void pal_col(char index, char color) {
   PAL_BUF[index & 0x1f] = color;