posix: options: mmap: disable xtensa support due to linker issue

The Xtensa linker scripts seem to be injecting syntax errors when
MMU is enabled. Disable the implication in Kconfig.mem for Xtensa
until linker issues are resolved.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

lib/posix/options/Kconfig.mem

@@ -27,7 +27,8 @@ config POSIX_SHARED_MEMORY_OBJECTS
 
 config POSIX_MAPPED_FILES
 	bool "POSIX memory-mapped files"
-	imply MMU
+	# disable Xtensa for now until linker issues are resolved
+	imply MMU if (CPU_HAS_MMU && !XTENSA)
 	help
 	  Select 'y' here and Zephyr will provide support for mmap(), msync(), and munmap().