Add strcmp to libc.

libc/src/string/CMakeLists.txt

add_entrypoint_object(
   SRCS
     strlen.cpp
   HDRS
     strlen.h
   DEPENDS
     libc.include.string
 )
 
+add_entrypoint_object(
+  strcmp
+  SRCS
+    strcmp.cpp
+  HDRS
+    strcmp.h
+  DEPENDS
+    libc.include.string
+)
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,
 # - Attach the REQUIRE_CPU_FEATURES property to the target,
 # - Add the fully qualified target to `${name}_implementations` global property for tests.
 function(add_implementation name impl_name)
   cmake_parse_arguments(
     "ADD_IMPL"

libc/test/src/string/CMakeLists.txt

add_libc_unittest(
   SUITE
     libc_string_unittests
   SRCS
     strlen_test.cpp
   DEPENDS
     libc.src.string.strlen
 )
 
+add_libc_unittest(
+  strcmp_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strcmp_test.cpp
+  DEPENDS
+    libc.src.string.strcmp
+)
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
   foreach(fq_config_name IN LISTS fq_implementations)
     get_target_property(required_cpu_features ${fq_config_name} REQUIRE_CPU_FEATURES)
     host_supports(can_run "${required_cpu_features}")
     if(can_run)
       add_libc_unittest(