Add extra cflags for LLVM_NEXT

The staging compiler update sometimes needs additional cflags to build, but those flags may not be recognised by the current compiler. Add a
new `llvmNextExtraCommonGlobalCflags` section and only append those
flags when LLVM_NEXT is set.

Test: LLVM_NEXT=true m
Bug: 236798112
Change-Id: Icc4687950acd44798b2cf09131a425ddfd919214