[libc++] Pin down the C version to use for SPEC

Also, disable a warning which allows additional benchmarks to build
successfully.

Author: ldionne

libcxx/test/benchmarks/spec.gen.py

@@ -45,7 +45,7 @@
     tune                 = base
     copies               = 1
     threads              = 1
-    CC                   = cc -O3
+    CC                   = cc -O3 -std=c18 -Wno-implicit-function-declaration
     CXX                  = {cxx} {compile_flags} {flags} {link_flags} -Wno-error
     CC_VERSION_OPTION    = --version
     CXX_VERSION_OPTION   = --version