build: compile V8 using system compiler

Stop using the copy of clang that is bundled with Google's build tools
and start using the compiler that is installed on the buildbots.
It stopped working on one of the machines because it looked in the
wrong place for system headers and is not representative of how users
build Node.js on their systems.

PR-URL: #17489
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>

Makefile

@@ -30,6 +30,7 @@ ifdef ENABLE_V8_TAP
   TAP_V8_BENCHMARKS := --junitout $(PWD)/v8-benchmarks-tap.xml
 endif
 
+V8_BUILD_OPTIONS += GYPFLAGS="-Dclang=0"
 V8_TEST_OPTIONS = $(V8_EXTRA_TEST_OPTIONS)
 ifdef DISABLE_V8_I18N
   V8_BUILD_OPTIONS += i18nsupport=off