[libc] disable syscall test without fullbuild

[michaelrj]

Summary

Our syscall implementation depends on a specific macro that's only
defined in our headers. If we're not using our headers, then the test
doesn't work. I've disabled the test in this case because there's no
point in testing the system libc's syscall implementation.

libc/test/src/unistd/CMakeLists.txt

add_libc_unittest(
   SUITE
     libc_unistd_unittests
   SRCS
     geteuid_test.cpp
   DEPENDS
     libc.src.unistd.geteuid
 )
 
+if(LLVM_LIBC_FULL_BUILD)
   add_libc_unittest(
     syscall_test
     SUITE
       libc_unistd_unittests
     SRCS
       syscall_test.cpp
     DEPENDS
       libc.src.unistd.__llvm_libc_syscall
       libc.include.errno
       libc.include.unistd
       libc.include.fcntl
       libc.include.sys_syscall
       libc.test.errno_setter_matcher
   )
+endif()