Integer getSDABase() {
		if (sdaBase != null) {
			if (sdaBase == -1) {
				return null;
			}
			return sdaBase;
		}
		sdaBase = getBaseOffset("_SDA_BASE_", ".sdata", ".sbss");
		if (sdaBase == -1) {
			getLog().appendMsg("ERROR: failed to establish _SDA_BASE_");
			return null;
		}
		setRegisterContext("r13", BigInteger.valueOf(sdaBase));
		return sdaBase;
	}
Get or establish _SDA_BASE_ value and apply as r13 context value to all memory blocks
with execute permission.
@return _SDA_BASE_ offset or null if unable to determine or establish
Ghidra/Processors/PowerPC/src/main/java/ghidra/app/util/bin/format/elf/relocation/PowerPC_ElfRelocationContext.java