bootstrap should set optimization level when calculating cflags

[comex]

bootstrap/builder.rs uses cc-rs to get a list of cflags for the target, which it then passes to cargo build as CFLAGS_<target>. As of rust-lang/cc-rs#294, the flags can depend on the optimization level: on iOS, with optimization disabled it passes -fembed-bitcode -fembed-bitcode-marker, but with optimization enabled it passes only -fembed-bitcode. However, bootstrap seems to always pass -fembed-bitcode-marker.

Note that some of the crates in the build, such as libcompiler_builtins and libbacktrace, use cc-rs themselves, causing a second set of cflags to be appended to the cc command line, identical except for respecting the optimization level. This seems odd, although it doesn't seem to cause any problems in itself.

Example build log:

OPT_LEVEL = Some("2")
TARGET = Some("aarch64-apple-ios")
CC_aarch64-apple-ios = Some("cc")
CFLAGS_aarch64-apple-ios = Some("-fPIC -arch arm64
-miphoneos-version-min=7.0 -isysroot
/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.3.sdk
-fembed-bitcode -fembed-bitcode-marker")
Detecting iOS SDK path for iphoneos
running: "cc" "-O2" "-fPIC" "-fPIC" "-arch" "arm64"
"-miphoneos-version-min=7.0" "-isysroot"
"/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.3.sdk"
"-fembed-bitcode" "-fembed-bitcode-marker" "-arch" "arm64"
"-miphoneos-version-min=7.0" "-isysroot"
"/Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.3.sdk"
"-fembed-bitcode" "-fno-builtin" "-fvisibility=hidden" "-ffreestanding"
"-fomit-frame-pointer" "-DVISIBILITY_HIDDEN" "-o"
"/tmp/build/x86_64-apple-darwin/stage2-std/aarch64-apple-ios/release/build/compiler_builtins-d5d18ec9ecb6ca67/out/../../libcompiler_builtins/compiler-rt/lib/builtins/absvdi2.o"
"-c" "../../libcompiler_builtins/compiler-rt/lib/builtins/absvdi2.c"