semihosting/arm-compat: replace heuristic for softmmu SYS_HEAPINFO

The previous numbers were a guess at best and rather arbitrary without
taking into account anything that might be loaded. Instead of using
guesses based on the state of registers implement a new function that:

 a) scans the MemoryRegions for the largest RAM block
 b) iterates through all "ROM" blobs looking for the biggest gap

The "ROM" blobs include all code loaded via -kernel and the various
-device loader techniques.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Andrew Strauss <astrauss11@gmail.com>
Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220225172021.3493923-18-alex.bennee@linaro.org>

include/hw/loader.h

@@ -343,4 +343,18 @@ int rom_add_option(const char *file, int32_t bootindex);
  * overflow on real hardware too. */
 #define UBOOT_MAX_GUNZIP_BYTES (64 << 20)
 
+typedef struct RomGap {
+    hwaddr base;
+    size_t size;
+} RomGap;
+
+/**
+ * rom_find_largest_gap_between: return largest gap between ROMs in given range
+ *
+ * Given a range of addresses, this function finds the largest
+ * contiguous subrange which has no ROMs loaded to it. That is,
+ * it finds the biggest gap which is free for use for other things.
+ */
+RomGap rom_find_largest_gap_between(hwaddr base, size_t size);
+
 #endif