[Test][Darwin] Disable test on watchos due to memory restraints (#135671)

Because of the nature of this test (mmap stress-test) it tests too large
of memory allocations to be stable on watchos. WatchOS has memory limits
that can lead to the termination of this process before it reaches the
limit set by the flag. Typical only on devices that are under heavy
load. disable on watchOS.

rdar://147222346

Author: thetruestblue

compiler-rt/test/asan/TestCases/Posix/mmap_limit_mb.cpp

@@ -1,5 +1,8 @@
 // Test the mmap_limit_mb flag.
 //
+// Unstable on watchOS devices under memory pressure.
+// UNSUPPORTED: watchos
+//
 // RUN: %clangxx_asan -O2 %s -o %t
 // RUN: %run %t 20 16
 // RUN: %run %t 30 1000000