Fix RISC-V stage2 compilation buffer overflow

The RISC-V target was experiencing segmentation faults during stage2
compilation when stdin was redirected. The root cause was insufficient
buffer size for the SOURCE buffer when compiling large programs like
shecc itself.

The total size of all included source files for RISC-V was approaching
the 512KB limit (MAX_SOURCE), and with stdin redirection adding
overhead, this caused buffer overflow and segmentation faults.

Increased MAX_SOURCE from 524288 (512KB) to 1048576 (1MB) to provide
adequate buffer space for self-hosting compilation on all architectures.

Author: jserv

src/defs.h

@@ -25,7 +25,7 @@
 #define MAX_BB_DOM_SUCC 64
 #define MAX_BB_RDOM_SUCC 256
 #define MAX_GLOBAL_IR 256
-#define MAX_SOURCE 524288
+#define MAX_SOURCE 1048576
 #define MAX_CODE 262144
 #define MAX_DATA 262144
 #define MAX_SYMTAB 65536