Cygwin: Test failure - winsup.api /systemcall.c

[thiru-mcw]

Description

• The systemcall test is failing at line no:30
• system("ls") fails because spawnvp cannot locate "/bin/sh".
• find_exec returns null as /bin/sh does not exist under C:.
• Workaround added to use C:\cygwin64\bin\sh.exe if /bin/sh is not executable.
• After path fix, waitpid sometimes returns 11 (SIGSEGV), showing process crash due to incomplete signal handling.

Acceptance criteria

• winsup.api/systemcall test passes in aarch64 cygwin build