[llvm-lit] Ulimit command not found in lit internal shell

[Harini0924]

The LLVM's lit internal shell fails to find the ulimit command and fails to set resource limits.
Error displayed:

# executed command: ulimit -s 1000
# .---command stderr------------
# | 'ulimit': command not found
# `-----------------------------
# error: command failed with exit status: 127

Files with failure:

• compiler-rt/test/asan/TestCases/Linux/allocator_oom_test.cpp
• compiler-rt/test/asan/TestCases/Posix/deep_call_stack.cpp
• compiler-rt/test/fuzzer/merge-posix.test
• compiler-rt/test/fuzzer/ulimit.test
• compiler-rt/test/hwasan/TestCases/print-memory-usage.c
• compiler-rt/test/msan/Linux/reexec_unlimited_stack.cpp