[FR] add branch protection option for ndk-build

[DanAlbert]

Rather than requiring

ifeq ($(TARGET_ARCH_ABI),arm64-v8a)
    libcxxabi_cppflags += -mbranch-protection=standard
endif

all over the place, we should add a LOCAL_BRANCH_PROTECTION := standard that is ignored for non-arm64.

[enh-google]

are you implying that we already have something similar for cmake, or should this bug cover cmake too?

[DanAlbert]

For CMake if there is no option yet we ought to be making an upstream change that isn't Android specific, so it's a bit more involved. Worth considering.

[jfgoog]

Some more info from Dan:

The goal is to not require users of the NDK to add libcxxabi_cppflags += -mbranch-protection=standard to their makefiles. Instead, they should be able to write LOCAL_BRANCH_PROTECTION := standard. Then ndk-build would internally ignore that for non arm64, and for arm64 would add -mbranch-protection=$OPTION_VALUE to LOCAL_CFLAGS internally.

[jfgoog]

Fixed in https://android-review.googlesource.com/q/I2c3879eceaba3712d0afa341dfa73af86afdde6a