[build] Add iOS detection

Add OS detection for iOS builds. If we are building for an iOS target,
the following V8 OS defines will be set:

V8_OS_BSD
V8_OS_MACOSX
V8_OS_POSIX
V8_OS_IOS  // This one is new.

The detection code is taken from Chromium's build_config.h file.

Bug: v8:9140
Change-Id: I435a8931dc0ae0eefdb893bc838a04470bcc57db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1569435Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60897}

include/v8config.h

@@ -64,6 +64,7 @@
 //  V8_OS_FUCHSIA       - Fuchsia
 //  V8_OS_LINUX         - Linux
 //  V8_OS_MACOSX        - Mac OS X
+//  V8_OS_IOS           - iOS
 //  V8_OS_NETBSD        - NetBSD
 //  V8_OS_OPENBSD       - OpenBSD
 //  V8_OS_POSIX         - POSIX compatible (mostly everything except Windows)
@@ -80,6 +81,9 @@
 # define V8_OS_BSD 1
 # define V8_OS_MACOSX 1
 # define V8_OS_POSIX 1
+# if defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
+#  define V8_OS_IOS 1
+# endif  // defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
 #elif defined(__CYGWIN__)
 # define V8_OS_CYGWIN 1
 # define V8_OS_POSIX 1