C toolchain: run SDCC-compiled C programs as gbos processes

- c/{gbos.h,crt0.s,libc.s,build.sh}: SDCC sm83 -> ROM-bank blob toolchain.
  libc syscall wrappers save/restore BC/DE/HL around rst $30 (trap clobbers
  them; SDCC expects them preserved).
- build via SDCC native asxxxx path (sdasgb/sdldgb), crt0 linked first so
  _start is the $4000 entry; blob INCBIN'd into a ROM bank.
- chello.c: prints a message + getpid() -> runs as 'chello' shell command.
- Makefile: auto-build c/*.bin, track as a dep of programs.o; gitignore blobs.
- README: document the C toolchain + the SDCC --asm=rgbds codegen bug that
  forced the native-toolchain approach.
- verified: '$ chello' -> 'hello from C on gbos!' / 'my pid is 2'.

1 files changed, 9 insertions, 0 deletions

diff --git a/c/chello.c b/c/chello.c
new file mode 100644
index 0000000..f570b61
--- /dev/null
+++ b/c/chello.c
@@ -0,0 +1,9 @@
+#include "gbos.h"
+void main(void) {
+    writes("hello from C on gbos!", 21);
+    nl();
+    writes("my pid is ", 10);
+    /* print pid as a digit */
+    { unsigned char c = getpid() + '0'; char d = c; writes(&d, 1); }
+    nl();
+}